rebase --apply: remove duplicated code

rebase --apply: remove duplicated code

Use move_to_original_branch() when reattaching HEAD after a fast-forward
rather than open coding a copy of that code. move_to_original_branch()
does not call reset_head() if head_name is NULL but there should be no
user visible changes even though we currently call reset_head() in that
case. The reason for this is that the reset_head() call does not add a
message to the reflog because we're not changing the commit that HEAD
points to and so lock_ref_for_update() elides the update. When head_name
is not NULL then reset_head() behaves like "git symbolic-ref" and so the
reflog is updated.

Note that the removal of "strbuf_release(&msg)" is safe as there is an
identical call just above this hunk which can be seen by viewing the
diff with -U6.

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