git-gui: Finished commit implementation.

git-gui: Finished commit implementation.

We can now commit any type of commit (initial, normal or merge) using
the same techniques as git-commit.sh does for these types of things.

If invoked as git-citool we run exit immediately after the commit was
finished.  If invoked as git-gui then we stay running.

Also fixed a bug which caused the commit message buffer to be lost
when the application shutdown and restarted.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>