rebase: fix --onto A...B parsing and add tests

rebase: fix --onto A...B parsing and add tests

The previous patch didn't parse "rebase --onto A...B" correctly when A
isn't an empty string. It also tried to be careful to notice a case in
which there are more than one merge bases, but forgot to give --all option
to merge-base, making the test pointless.

Fix these problems and add a test script to verify. Improvements to the
script to parse A...B syntax was taken from review comments by Johannes
Schindelin.

Signed-off-by: しらいし ななこ <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>