Basic update algorithm:

 1. recurse to all of our direct dependencies and
    update their bases and branches

 2. Update our base.

    i. Compute our base's set of desired included branches:
         The set of desired included branches for a base
         is the union of the desired included branches for each
         branch named in the base's branch's direct deps, plus
         the name of every direct external dep.
         The set of desired included branches for a branch
         is the set of desired included branches for the branch's
         base plus the branch name itself.

    ii. For each source in the best order, do the following merge:
        (Our base has sources:
            - the branch for each direct dep
            - the remote base)

       Find the common ancestor.

       Check for unwanted dependency removals.  
       An unwanted dependency removal is
          A branch in the desired included branches
          Which exists in the common ancestor's actual included branches
          but which is missing in the source's actual included branches
          (NB that this definition excludes dependency removals
          which have already occurred on our base; these will be
          reverted later.)
       For each unwanted dependency removal (ie for each such
       branch), find the most recent commit which unwantedly removed
       the dep from the source's actual included branches ("relevant
       unwanted removal commit").  (Abort if any such commit is a
       merge.)  Select the earliest relevant unwanted removal commit
       (from the set of relevant unwanted removal commits
       corresponding to the unwanted dependency removals).
       Merge from the ancestor of the relevant unwanted removal commit.
       Merge from the relevant unwanted removal commit using -s ours.

       (The purpose of this, and the result, is that the unwanted
       dependency removal has gone away.  Doing things in this order
       tries to keep the unwanted dependency removal's reversions as
       close as possible to their originating points.  The
       recursion, which processes dependencies before their clients,
       tries to keep the reversion churn localised: client branches
       of a branch affected by an unwanted removal will benefit from
       that client's resolution of the situation.)

       Now continue to the next unwanted dependency removal.

       If there are no (more) unwanted dependency removals, merge
       from the source.

    iii.
       Check for missing or unwanted dependency inclusions.  Compare
       our base's desired included branches with our base's actual
       included branches.  In exceptional conditions, they will not
       be identical.  This can happen, for example, because a
       dependency removal was incorporated into our base branch but
       the removed branch was introduced as an explicit dependency.
       This will also happen if we remove a dependency ourselves.

       Do the unwanted inclusions first, and then the missing ones.
       (This is because that the missing ones cannot depend on the
       unwanted ones, as otherwise the unwanted ones would be in the
       desired inclusions.  So removing the unwanted ones first
       reduces the chances of conflicts.)

       So, repeatedly:
         * Do the comparison between desired and actual included
         * Pick a missing inclusion, or failing that an unwanted one
           (call this the "relevant" branch)
         * Depth first search all of the dependencies of the
           relevant branch; if any of these is also a missing
           (resp. unwanted) inclusion, start again processing it
           instead.
         * Attempt to apply the appropriate diff to add (resp. remove)
           the contents of the relevant patch (adjusted appropriately
           for metadata, XXX??? particularly the actual inclusion list)
         * Go round again looking for another discrepancy.

 3. Update our branch.
       Our branch has sources:
          - our base
          - the remote for our branch
       For each source in the best order, do the merge.

       Double-check the actual dependency inclusions.  In
       particular, if we just upgraded to actual dependency tracking
       we may need to explicitly add our branch name to the actual
       dependency inclusion list.

The "best order" for merges is in order of recency of common
ancestor, most recent first, and if that does not distinguish,
merging from local branches first.
     
"Recency" refers to the order from git-rev-list --date-order.

Actual included branches:

  This is tracked explicitly in .topbloke/included, one branch per
  line.  For compatibility with older versions, every time we think
  about a base, branch or source above, we check whether
  .topbloke/included is present.

  If it isn't then we calculate a child commit which has a
  .topbloke/included.  In the case of a remote branch or base, we
  substitute this child commit for the relevant remote ref but do
  not record it in the remote ref; in the case of a local branch or
  base, we advance the local branch or base accordingly.

  When .topbloke/included is calculated in this way, it always gets
  the list of desired included branches.  (topgit,
  which does not support dependency deletion, always has exactly the
  desired branches actually included.)

  Non-topbloke-controlled branches are never recorded in included.


Branch removal:

 - removed branch must be removed from the deps of its
   ex-children, and replaced with the deps of the removed
   branch

 - removed branch wants not to be in list of branches "git-branch"
   et al any more
      branches in refs/top-branches ?  yes
      deleted branches do something to the base ?  no

 deleting empty branch: dependencies fine
 deleting nonempty branch: if any dependencies found, their
    updates break

 purpose of deleting?
   - remove from views of active branches
   - prevent new commits
   - remove from dependencies of active branches
       only makes sense if no active branches depend on it.

 undeleting
   - just unmark the branch as deleted


Foreign branches:
 When merging from a foreign dependency, check that it
 does not have .topbloke/included or .topbloke/flags; if it
 does, could produce a new commit which has .topbloke removed
 and merge from that

Branch naming:
 needs to be globally unique
 so put email address in it
tg operations search for applicable branches
safe charset for branch names
  . - / 0-9 a-z
not permitted (git-check-ref-format(1))
 spc ~ ^ : ? * [ \
 @{ .lock.
 (apropos of shell)


When pulling, which remotes get to update which branches ?
Complicated question!

For now, have "blessed" remotes, which we always pull and update from.
All these count as sources above.

Update operation restrictions available, which restrict use of various
sources above ?  What about implications for correctness of merge
algorithm ?


Concept of a "stack" ?
Unnecessary - instead, deal with leaf branches
Operations like "go up the stack", goes towards leaf.  Hopefully unique.
"Down" the stack, uses a "conventional" linearisation
Stack reordering op ?  auto adjust deps