dgit: --clean=dpkg-source: Check for untracked unignored files

dgit: --clean=dpkg-source: Check for untracked unignored files

We would like to spot if the user forgot to `git add' a file.
This can be done by calling clean_tree_check_git, after rules
clean (if applicable).

We need to make this configurable.  We do so via the clean mode, with
a comma-separated checking control suffix (and short aliases) like we
did for --clean=git,ignores.

The default should be cautious, ie to do this check, but often the
user will want to disable it because the source package has a buggy
clean target or no or insufficient .gitignore.  Existing users should
probably get the new check until they choose otherwise (which we have
made easier for them with the .clean-mode-newer config option).

So we change the meanings of -wd and -wdd to include the new check,
and provide new build modes ...,no-check aka -wdn / -wddn to disable
it.

To implement this we introduce a new clean_tree_check_git_wd function
to do the actual work, particularly because both during cleaning and
cleanliness checking, we want to print some hints to the user if the
check fails.

We can't do the new check if we applied patches dirtily to run the
rules target, because it will trip over the result of patch
application.  This way of working is just too poor to support this new
check.

The test suite generally tests the default versions, not the no-check
versions.  We must teach the test to expect the new check.  This is
most easily done with a separate case for the check side of the -wd
clean modes.  And we need to support the no-check variant too,
because:

The push-source-with-changes test does in fact work with a built tree
and needs to test the no-check variant.

The gbp tests sometimes involve patch application.  Rather than trying
to predict which of them do (in which cases there would be no clean
check), we force them all to ,no-check.

The oldnewtagalt test can use -wgf.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>