gitk: Only restore window size from ~/.gitk, not position

gitk: Only restore window size from ~/.gitk, not position

This also limits the window size to the screen size.  That is better
than nothing, but it isn't perfect, since ideally we would take into
account window decorations, and things such as gnome panels or the
Mac OS X dock and menu bar, but I don't know how to do that.

On Cygwin this is as good as restoring the whole geometry (size and
position) at working around the Cygwin Tk bugs, according to Mark
Levedahl.

Tested-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>