rebase: store orig_head as a commit

rebase: store orig_head as a commit

Using a struct commit rather than a struct oid to hold orig_head means
that we error out straight away if the branch being rebased does not
point to a commit. It also simplifies the code that handles finding
the merge base and fork point as it no longer has to convert from an
oid to a commit.

To avoid changing the behavior of "git rebase <upstream> <branch>" we
keep the existing call to read_ref() and use lookup_commit_object()
on the oid returned by that rather than calling
lookup_commit_reference_by_name() which applies the ref dwim rules to
its argument.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>