branch: advice using git-help(1) instead of man(1)

branch: advice using git-help(1) instead of man(1)

8fbd903e (branch: advise about ref syntax rules, 2024-03-05) added
an advice about checking git-check-ref-format(1) for the ref syntax
rules. The advice uses man(1). But git(1) is a multi-platform tool and
man(1) may not be available on some platforms. It might also be slightly
jarring to see a suggestion for running a command which is not from
the Git suite.

Let’s instead use git-help(1) in order to stay inside the land of
git(1). This also means that `help.format` (for `man`, `html` or other
formats) will be used if set.

Also change to using single quotes (') to quote the command since that
is more conventional.

While here let’s also update the test to use `{SQ}`, which is more
readable and easier to edit.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>