contrib/subtree: add -S/--gpg-sign
Allows optionally signing the commits that git subtree creates. This can
be necessary when working in a repository that requires gpg signed
commits.
Signed-off-by: Patrik Weiskircher <patrik@pspdfkit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
authored by