rebase topology tests: fix commit names on case-insensitive file systems

The recently introduced tests used uppercase letters to denote
cherry-picks of commits having the corresponding lowercase letter names.
The helper functions also set up tags with the names of the commits.

But this constellation fails on case-insensitive file systems because
there cannot be distinct tags with names that differ only in case.

Use a less subtle convention for the names of cherry-picked commits.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>