tests: avoid variations of the `master` branch name

tests: avoid variations of the `master` branch name

The term `master` has a loaded history that serves as a constant
reminder of racial injustice. The Git project has no desire to
perpetuate this and already started avoiding it.

The test suite uses variations of this name for branches other than the
default one. Apart from t3200, where we just addressed this in the
previous commit, those instances can be renamed in an automated manner
because they do not require any changes outside of the test script, so
let's do that.

Seeing as the touched branches have very little (if anything) to do with
the default branch, we choose to use a completely separate naming
scheme: `topic_<number>` (it cannot be `topic-<number>` because t5515
uses the `test_oid` machinery with the term, and that machinery uses
shell variables internally, whose names cannot contain dashes).

This trick was performed by this (GNU) sed invocation:

	$ sed -i 's/master\([a-z0-9]\)/topic_\1/g' t/t*.sh

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