subrepo: lazily update git's local tracking branches
This continues the strategy of separation between hg pull and hg update in
git subrepos by only dealing with git's branches on an update. This behavior
tries to cover the bare essentials of the semantics of git pull in the subrepo
when the parent repo does hg pull and hg update.
$ "$TESTDIR/hghave" execbit || exit 80
$ hg init repo
$ cd repo
$ echo foo > foo
$ chmod 644 foo
$ hg ci -qAm '644'
$ chmod 755 foo
$ hg ci -qAm '755'
reverting to rev 0
$ hg revert -a -r 0
reverting foo
$ hg st
M foo
$ hg diff --git
diff --git a/foo b/foo
old mode 100755
new mode 100644