git-gui: Honor system font and let user configure fonts.

git-gui: Honor system font and let user configure fonts.

Rather than hardcoding our fonts to something that I thought was
reasonable, guess font_ui off the font used by the system in the
menu bar.  This way the application conforms by default to whatever
the user's desktop is setup to.

We also now let the user supply font configuration through their
repository configuration as gui.fontui (the overall UI font) and
gui.fontdiff (the font used for the diff viewer).

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