git-svn: add --username/commit-url options for branch/tag
git-svn: add --username/commit-url options for branch/tag

Add ability to specify on the command line the username to perform the
operation as and the writable URL of the repository to perform it on.

[ew: shortened subject]

Signed-off-by: Igor Mironov <65f78dcd0534c7a1d001a431f12d03a1b819f5b5@gmail.com>
Acked-by: Eric Wong <8c2951295f8292b0c03727bc4652981172298eae@yhbt.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git