provide more errors for the "merge into empty head" case
provide more errors for the "merge into empty head" case

A squash merge into an unborn branch could be implemented by building the
index from the merged-from branch, and doing a single commit, but this is
not supported yet.

A non-fast-forward merge into an unborn branch does not make any sense,
because you cannot make a merge commit if you don't have a commit to use
as the parent.

Signed-off-by: Paolo Bonzini <67a772724e2fbc123894e2d469547b8302dd9fd5@gnu.org>
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