git-gui: heed core.commentChar/commentString

git-gui: heed core.commentChar/commentString

This amends 1ae85ff6d (git-gui: strip comments and consecutive empty
lines from commit messages, 2024-08-13) to deal with custom comment
characters/strings.

The magic commentString value "auto" is not handled, because the option
makes no sense to me - it does not support comments in templates and
hook output, and it seems far-fetched that someone would introduce
comments during editing the message.

Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>