t5526: avoid depending on a specific default branch name

t5526: avoid depending on a specific default branch name

While at it, use different default branch names for the three different
repositories involved in the test script: this makes it easier to debug
failures, too (otherwise you have to wonder which `master` branch was
meant: the super project's? The submodule's? The nested submodule's?).

Note: this touches code that was originally modified to prepare for
renaming the default branch name to `main`. This patch side-steps that
effort completely by overriding the initial branch name explicitly.

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