git.el: Fix git-amend-commit to support amending an initial commit.
git.el: Fix git-amend-commit to support amending an initial commit.

Signed-off-by: Alexandre Julliard <a3c1c71ac0d85b781251f68481a696b8e4d0781d@winehq.org>

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