t3427: condense the unnecessarily repetitive test cases into three

t3427: condense the unnecessarily repetitive test cases into three

Previously, this test script performed essentially three rebases and
verified breakages by testing the post-rebase commits' messages.

To do so, the rebases were performed multiple times, though, once per
commit message to test. This wastes electricity (and CO2) and time.

Let's condense the test cases to the essential number: the number of
different rebases to validate.

On Windows, where the scripted nature of the `--preserve-merges` backend
hurts performance rather badly, this reduces the overall runtime in this
developer's setup from ~1m to ~28s while still performing the exact same
testing as before.

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