Speed up zgit_isworktreeclean by a factor of 5

Use ls-files --modified instead of diff --quiet.
zgit_isworktreeclean: ls-files on $gitdir, not pwd
Another isworktreeclean fix
Simplify grep | sed into an awk call

Hopefully this will work better for OS X also.

Yes, I could've "just" simplified the regex to not use extended
features, but I'd rather see if I can get away with this instead.
awk magic fails if the repository has a space in the name. Replace with proper git-svn command, since it's not unreasonable to assume git-svn support when this code is invoked.
Merge remote branch 'jcorbin/master'