git-svn: do a partial rebuild if rev_map is out-of-date

git-svn: do a partial rebuild if rev_map is out-of-date

Suppose you're using git-svn to work with a certain SVN repository.
Since you don't like 'git-svn fetch' to take forever, and you don't want
to accidentally interrupt it and end up corrupting your repository, you
set up a remote Git repository to mirror the SVN repository, which does
its own 'git-svn fetch' on a cronjob; now you can 'git-fetch' from the
Git mirror into your local repository, and still dcommit to SVN when you
have changes to push.

After you do this, though, git-svn will get very confused if you ever
try to do 'git-svn fetch' in your local repository again, since its
rev_map will differ from the branch's head, and it will be unable to
fetch new commits from SVN because of the metadata conflict.  But all
the necessary metadata are there in the Git commit message; git-svn
already knows how to rebuild rev_map files that get blown away, by
using the metadata.

This patch teaches git-svn do a partial rebuild of the rev_map to
match the true state of the branch, if it ever is used to fetch again.

This will only work for projects not using either noMetadata or
useSvmProps configuration options; if you are using these options,
git-svn will fall back to the previous behaviour.

Signed-off-by: Deskin Miller <deskinm@umich.edu>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>