git-gui: remove lines starting with the comment character

git-gui: remove lines starting with the comment character

The comment character is specified by the config variable
'core.commentchar'. Any lines starting with this character is considered
a comment and should not be included in the final commit message.

Teach git-gui to filter out lines in the commit message that start with
the comment character using git-stripspace. If the config is not set,
'#' is taken as the default. Also add a message educating users about
the comment character.

Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>