t7064: avoid relying on a specific default branch name

t7064: avoid relying on a specific default branch name

To allow us to consider a change in the default behavior of `git init`
where it uses a more inclusive name for the initial branch, we must
first teach the test suite not to rely on a specific default branch
name. In this patch, we teach t7064 that trick.

To that end, we set a specific name for the initial branch. Ideally, we
would simply start out by calling `git branch -M initial-branch`, but
there is a bug in `git branch -M` that does not allow renaming branches
unless they already have commits. This will be fixed in the
`js/init-defaultbranch-advice` topic, and until that time, we use the
equivalent (but less intuitive) `git checkout -f --orphan`.

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