The python git module may or may not be enabled, allow this code to
function without it, falling back to the same method as metadata_scm.bbclass
uses. This will be cleaned up in the next round of feature development.
(From OE-Core rev: 6350586ba9f4a4107a2d457590824cd4d662d5b9)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>