tests(git-p4): transition to the default branch name `main`

tests(git-p4): transition to the default branch name `main`

In the previous commits, we adjusted the test suite to use the branch
name `main` for initial branches.

The `git p4`-related tests are a bit harder to adjust because `git p4`
uses the ref `refs/heads/p4/master` to track the remote branches, and
for now, we do not want to change that (this might be the subject of a
future patch series). We only need to adjust for the actual initial
branch name to be changed to `main`.

This allows us to define `GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main`
for those tests.

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