interpret-trailers: allow running outside a repository
interpret-trailers: allow running outside a repository

It may be useful to run git-interpret-trailers without needing to be in
a repository.

Signed-off-by: John Keeping <a51dda7c7ff50b61eaea0444371f4a6a9301e501@keeping.me.uk>
Acked-by: Christian Couder <58cc993db606fd7043084d51c23db751557f1001@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git