git-svn: add support for SVN::Mirror/svk using revprops for metadata

git-svn: add support for SVN::Mirror/svk using revprops for metadata

Pass --use-svm-props or set the svn.usesvmprops key with git-config
to enable using properties set by SVN::Mirror when it mirrored the
upstream URL.

This is heavily based on work from Sam Vilain:
> From: Sam Vilain <sam@vilain.net>
> Date: Sun, 11 Feb 2007 12:34:45 +1300
> Subject: [PATCH] git-svn: re-map repository URLs and UUIDs on SVK mirror paths
>
> If an SVN revision has a property, "svm:headrev", it is likely that
> the revision was created by SVN::Mirror (a part of SVK).  The property
> contains a repository UUID and a revision.  We want to make it look
> like we are mirroring the original URL, so introduce a helper function
> that returns the original identity URL and UUID, and use it when
> generating commit messages.

Signed-off-by: Eric Wong <normalperson@yhbt.net>