gitk: persist position and size of the Tags and Heads window

gitk: persist position and size of the Tags and Heads window

The Tags and Heads window always opens at a default position and size,
requiring users to reposition it each time. Remember its geometry
between sessions in the config file as `geometry(showrefs)`.

Note that the existing configuration is sourced in proc savestuff
right before new settings are written. This makes the old settings
available as local variables(!) and does not overwrite the current
settings. Since we need access to the global geometry(showrefs), it
is necessary to unset the local variable.

Helped-by: Michael Rappazzo <rappazzo@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>