git-gui: add horizontal scrollbar to commit buffer

While the commit message widget has a configurable fixed width, it
nevertheless allowed to write commit messages which exceeded this limit.
Though there is no visual clue, that there is scrolling going on. Now
there is a horizontal scrollbar.

There seems to be a bug in at least Tcl/Tk up to version 8.6.8, which
does not update the horizontal scrollbar if one removes the whole
content at once.

Suggested-by: Birger Skogeng Pedersen <birger.sp@gmail.com>
Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>