Use git-rev-parse to find the local GIT repository

Use git-rev-parse to find the local GIT repository

Use the latest git-rev-parse technology to allow some StGIT commands to
function correctly in subdirectories of the working directory.

Any command that relies on git-read-tree still doesn't work (changes to
GIT forthcoming).

Signed-off-by: Chuck Lever <cel@netapp.com>