git-gui: do not end the commit message with an empty line

git-gui: do not end the commit message with an empty line

The commit message is processed to remove unnecessary empty lines.
In particular, it is ensured that the text ends with at most one LF
character. This one is always present, because the Tk text widget
ensures that is present.

However, did not consider that the processed text is written to the
commit message file using `puts`, which also appends a LF character,
so that the final commit message ends with two LF. Trim all trailing
LF characters, and while we are here, use `string trim`, which lets
us remove the leading LF in the same command.

Reported-by: Gareth Fenn <garethfenn@gmail.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>