t5503: prepare aligned comment for replacing `master` with `main`

In an upcoming commit, we will use `main` as the default branch name in t5503 instead of `master`. This will require extra padding in ASCII-art commit graphs, which we hereby add preemptively. By doing this preemptively rather than after the commit applying the search-and-replace, it is more obvious that we caught all aligned comments that are affected by the latter commit. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>