classes/patch: when PATCHTOOL = "git" double-check the repository

classes/patch: when PATCHTOOL = "git" double-check the repository

If a bug is present or the user has set PATCHTOOL = "git" on a source
tree that isn't git, if we try to perform git operations (such as
committing or changing branches) when extracting source, then we might
in fact be running those operations on the metadata repository if the
build directory is underneath, say, poky or OE-Core, and that could
make a mess. Check if the source tree is a git repository and refuse
to continue if it isn't.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>