git-svn: dcommit should diff against the current HEAD after committing

This is a followup to dd31da2fdc199132c9fd42023aea5b33672d73cc.
Regardless of whether we commit an alternate head, we always
diff-tree based on the current HEAD, and rebase against our
remote reference as necessary.

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