git-svn: add git svn propget

git-svn: add git svn propget

This allows one to easily retrieve a single SVN property from within
git-svn without requiring svn or remembering the URL of a repository

	* git-svn.perl (%cmd): Add the new command `propget'.
	($cmd_dir_prefix): New global.
	(&get_svnprops): New helper.
	(&cmd_propget): New.  Use &get_svnprops.
	* t/t9101-git-svn-props.sh: Add a test case for propget.

[ew: make sure the rev-parse --show-prefix call doesn't break
     the `git-svn clone' command]

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>