t5323: prepare centered comment for `master` -> `main`

t5323: prepare centered comment for `master` -> `main`

We are about to search-and-replace all mentions of `master` in t5323 by
`main`, which is two characters shorter. To prepare for that, let's add
padding to centered lines that will make them briefly uncentered, but
will be re-centered in the commit that performs that rename.

Doing it this way (instead of padding after replacing) makes it easier
to verify the validity of the patch that replaces `master` by `main`.

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