pgm: Report not in git repository rather than NPE

If we can't find a Git repository after searching all the way up
to the filesystem root, JGit threw an NPE because we tried to get
the path of null.

Change-Id: I4e42364aeba53993c0ea528a9aeba3f08c7b3321
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Reviewed-by: Robin Rosenberg <robin.rosenberg@dewire.com>