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

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>