git-gui: Minor options dialog UI cleanups.

git-gui: Minor options dialog UI cleanups.

Display the name of "this" repository rather than the quite ambiguous
string "This".  The idea is that seeing the name of the directory the
repository is stored in should help jog the user's memory about what
they are setting options for.

Also place the options dialog immediately over the git-gui main window
when it gets opened.  This way the user isn't scrolling very far away
to gain access to the window.  At least on my Mac OS X system not doing
this makes the options dialog open rather far away, thus requiring lots
of mouse activity to reach it.

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