git-svn: do our best to ensure that our ref and rev_db are consistent

git-svn: do our best to ensure that our ref and rev_db are consistent

Defer any signals that cause termination while they are
updating; and put the update-ref call as close to the rename()
as possible.  Also, make things extra-safe (but slower) for
people using --no-metadata since they can't rely on .rev_db
being rebuilt if it's clobbered (well, I'm calling update-ref
with the -m flag for reflogs, we don't yet have a way to rebuild
.rev_db from reflogs.

Signed-off-by: Eric Wong <normalperson@yhbt.net>