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 <email@example.com>