git-svn: add the 'dcommit' command

git-svn: add the 'dcommit' command

This is a high-level wrapper around the 'commit-diff' command
and used to produce cleaner history against the mirrored repository
through rebase/reset usage.

It's basically a more polished version of this:

for i in `git rev-list --no-merges remotes/git-svn..HEAD | tac`; do
	git-svn commit-diff $i~1 $i
done
git reset --hard remotes/git-svn

Signed-off-by: Eric Wong <redacted>
Signed-off-by: Junio C Hamano <redacted>