subtree: allow --squash to be used with --rejoin

Besides being a genuinely useful thing to do, this also just makes sense
and harmonizes which flags may be used when.  `git subtree split
--rejoin` amounts to "automatically go ahead and do a `git subtree
merge` after doing the main `git subtree split`", so it's weird and
arbitrary that you can't pass `--squash` to `git subtree split --rejoin`
like you can `git subtree merge`.  It's weird that `git subtree split
--rejoin` inherits `git subtree merge`'s `--message` but not `--squash`.

Reconcile the situation by just having `split --rejoin` actually just
call `merge` internally (or call `add` instead, as appropriate), so it
can get access to the full `merge` behavior, including `--squash`.

Signed-off-by: Luke Shumaker <lukeshu@datawire.io>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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