git-svn: dcommit/rebase confused by patches with git-svn-id: lines

git-svn: dcommit/rebase confused by patches with git-svn-id: lines

When patches are merged from another git-svn managed branch,
they will have the git-svn-id: metadata line in them (generated
by git-format-patch).

When doing rebase or dcommit via git-svn, this would cause
git-svn to find the wrong upstream branch.  We now verify
that the commit is consistent with the value in the .rev_db
file.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>