git-gui: Paper bag fix quitting crash after commit

git-gui: Paper bag fix quitting crash after commit

My earlier introduction of the GITGUI_BCK file (which saves the user's
commit message buffer while they are typing it) broke the Quit function.
If the user makes a commit we delete the GITGUI_BCK file; if they then
immediately quit the application we fail to rename the GITGUI_BCK file
to GITGUI_MSG.  This is because the file does not exist, but our flag
still says it does.  The root cause is we did not unset the flag during
commit.

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