t3200: avoid variations of the `master` branch name

t3200: avoid variations of the `master` branch name

To avoid branch names with a loaded history, we already started to avoid
using the name "master" in a couple instances.

The `t3200-branch.sh` script uses variations of this name for branches
other than the default one. So let's change those names, as
"lowest-hanging fruits" in the effort to use more inclusive naming
throughout Git's source code. While at it, make those branch names
independent from the default branch name.

In this particular instance, this rename requires a couple of
non-trivial adjustments, as the aligned output depends on the maximum
length of the displayed branches (which we now changed), and also on the
alphabetical order (which we now changed, too).

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