builtin rebase: support `git rebase --onto A...B`

builtin rebase: support `git rebase --onto A...B`

This commit implements support for an --onto argument that is actually a
"symmetric range" i.e. `<rev1>...<rev2>`.

The equivalent shell script version of the code offers two different
error messages for the cases where there is no merge base vs more than
one merge base.

Though it would be nice to retain this distinction, dropping it makes it
possible to simply use the `get_oid_mb()` function. Besides, it happens
rarely in real-world scenarios.

Therefore, in the interest of keeping the code less complex, let's just
use that function, and live with an error message that does not
distinguish between those two error conditions.

Signed-off-by: Pratik Karki <predatoramigo@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>