rebase -m: don't fork git checkout

rebase -m: don't fork git checkout

Now that reset_head() can handle the initial checkout of onto
correctly use it in the "merge" backend instead of forking "git
checkout".  This opens the way for us to stop calling the
post-checkout hook in the future. Not running "git checkout" means
that "rebase -i/m" no longer recurse submodules when checking out
"onto" (thanks to Philippe Blain for pointing this out). As the rest
of rebase does not know what to do with submodules this is probably a
good thing. When using merge-ort rebase ought be able to handle
submodules correctly if it parsed the submodule config, such a change
is left for a future patch series.

The "apply" based rebase has avoided forking git checkout
since ac7f467fef ("builtin/rebase: support running "git rebase
<upstream>"", 2018-08-07). The code that handles the checkout was
moved into libgit by b309a97108 ("reset: extract reset_head() from
rebase", 2020-04-07).

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