git-svn: reinstate old rev_db optimization in new rev_map

git-svn: reinstate old rev_db optimization in new rev_map

This reinstates an old optimization in .rev_db which
stored the highest revision number we scanned, allowing
us to avoid scanning the SVN log for those revisions
again in a subsequent invocation.

This means the last 24-byte record in a .rev_map file
can be a 4-byte SVN revision number with 20-bytes of
zeroes representing a non-existent commit.  This record
can and will be overwritten when a new commit iff
the commit is all zeroes.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>