git-svn: add git svn proplist

This allows one to easily retrieve a list of svn properties from within
git-svn without requiring svn or knowing the URL of a repository.

	* git-svn.perl (%cmd): Add the command `proplist'.
	(&cmd_proplist): New.
	* t/t9101-git-svn-props.sh: Test git svn proplist.

Signed-off-by: Benoit Sigoure <tsuna@lrde.epita.fr>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>