git-svn: add ability to specify --commit-url for dcommit

git-svn: add ability to specify --commit-url for dcommit

This allows one to use public svn:// URLs for fetch and
svn+ssh:// URLs for committing (without using the complicated
rewriteRoot option, reimporting or git-filter-branch).

Using this can also help avoid unnecessary server
authentication/encryption overhead on busy SVN servers.

Along with the new --revision option, this can also be allowed
to override the branch detection in dcommit, too.  This is
potentially dangerous and not recommended!  (And also purposely
undocumented, but the loaded gun is there in case somebody
wants to make it safe).

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