Topbloke branch is:
	- branch in refs/topbloke-tips/<full-name>
		contains the working version, fast-forwarding
	- branch in refs/topbloke-bases/<full-name>
		contains the base version, being the pulling
		merge of all the bases

In-tree, there are metadata files in .topbloke

	msg		brach "commit message"

	deps		direct dependencies, one per line
			as either:
				topbloke branch name
				/refs/heads/<something>

	included	actual included branches, one per line
				topbloke branch name

	flags		flags that apply to this branch, one per line
			 unknown flags starting with [a-z] are ok;
			 otherwise fatal.  Currently defined flags:
				Deleted		branch is deleted


<full-name> has the format:
	<email>@<domain.name>/<yyyy>-<mm>-<dd>T<hh><mm><ss>Z/<nickname-path>
where nickname-path's first component must not start with a digit 
contain ~ or @  (: is not allowed in refs hence the squashing)
eg
	ijackson@chiark.greenend.org.uk/2012-01-20T225127Z/reorg/sponge
NB only exactly that date format is allowed and timezone must be Z.

Branches may be specified as
	[<qualifier>/...]<nickname-path-spec>
where <qualifier>/ is one of
	[<email>]@[<domain.name>/
		Only branches matching the specified email local part
		or domain name match
	[<date-spec>/]
		A prefix of the ISO8601 date spec, stopping
		just after a numeric component (or at the end)
	[<approx-date-containing-~>/]
		Intepreted by date -d after ~s have been replaced by
		spaces.  When we come to select, take the branch
		nearest that date rather than the most recent


<nickname-path-spec> may be
	<nickname-path>
	./<nickname-new-leaf-component>
	../<nickname-path-spec>

<date-spec> may be
	<approximate date spec containing at least one ~>
		means all branches are candidates

So overall, if the current branch is
	ijackson@chiark.greenend.org.uk/2011-08-20T120320Z/fixes/pudding
then all of the following can refer to the same branch
	./sponge
	../reorg/sponge
	reorg/sponge
	2012/reorg/sponge
	2012/reorg/sponge
	2012/./sponge
	jan~/./sponge
	ijackson@/reorg/sponge
	ijackson@/2012/reorg/sponge
	ijackson@chiark.greenend.org.uk/reorg/sponge
	ijackson@chiark.greenend.org.uk/2012-01-20T225127Z/reorg/sponge

Search algorithm:
	1. prefer branches with email address (or domain)
	   matching current branch
	2. prefer branches with our own email address (or domain)
	3. other branches
Within this there may be branches with multiple dates; prefer the most
recent.