In the near future, we want to change Git's default branch name to
`main`. In preparation for that, stop using it as a branch name in the
test suite. Replace that branch name by `topic`, the same name we used
to rename variations of `master` in b6211b89 (tests: avoid variations
of the `master` branch name, 2020-09-26).
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>