git-gui: strip comments and consecutive empty lines from commit messages

git-gui: strip comments and consecutive empty lines from commit messages

This is also known as "washing". This is consistent with the behavior of
interactive git commit, which we should emulate as closely as possible
to avoid usability problems. This way commit message templates and
prepare hooks can be used properly, and comments from conflicted rebases
and merges are cleaned up without having to introduce special handling
for them.

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