Not yet used, but essential for resuming cleanly.
Note that, in normmal operation, only one commit is made to export.log
during an export; the incomplete version only gets to the journal and
is then overwritten.
This commit was supported by the NSF-funded DataLad project.
So it will be available later and elsewhere, even after GC.
I first though to use git update-index to do this, but feeding it a line
with a tree object seems to always cause it to generate a git subtree
merge. So, fell back to using the Git.Tree interface to maniupulate the
trees, and not involving the git-annex branch index file at all.
This commit was sponsored by Andreas Karlsson.
