pull: implement pulling into an unborn branch

pull: implement pulling into an unborn branch

b4dc085 (pull: merge into unborn by fast-forwarding from empty
tree, 2013-06-20) established git-pull's current behavior of pulling
into an unborn branch by fast-forwarding the work tree from an empty
tree to the merge head, then setting HEAD to the merge head.

Re-implement this behavior by introducing pull_into_void() which will be
called instead of run_merge() if HEAD is invalid.

Helped-by: Stephen Robin <stephen.robin@gmail.com>
Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>