t5703: adjust a test case for the upcoming default branch name

We want to rename the default branch name used by `git init` in the near future, using `main` as the new name. In preparation for that, we adjust a test case that wants to rename the default branch to a different name that however has the same length. We use `none` as that name because it matches the length of `main`. As this test case cannot possibly pass until the default branch name is _actually_ changed, we temporarily guard it behind a special-purpose prereq, until the test suite is fully converted to use that new default branch name. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>