git-gui: Simplified format of geometry configuration.

git-gui: Simplified format of geometry configuration.

The gui.geometry config value was starting to contain
the odd string \\{ as part of its value due to the
way the Tcl lists were being supplied to git repo-config.

Now we write out only three values: the overall window
geomtry, the y position of the horizontal sash, and
the x position of the vertical sash.  All other data is
skipped, which makes the gui.geometry value simpler.

While debugging this I noticed that the save_my_config
procedure was being invoked multiple times during exit
due to do_quit getting invoked over and over again.  So
now we set a flag in do_quit and don't perform any of our
"at exit" type of logic if we've already been through the
do_quit procedure once.

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