gitk: Use symbolic font names "sans" and "monospace" when available

gitk: Use symbolic font names "sans" and "monospace" when available

The following only concerns systems using X and the client-side font
rendering framework from freedesktop.org.  Windows and Mac OS X are
not affected.

Starting with version 8.5, Tk uses freetype and fontconfig by default
to render fonts on platforms that support it.  Gitk currently defaults
to the font Helvetica for the interface and Courier for diffs, and
both unfortunately look rather bad on screen in the default
configuration on many Linux distros with anti-aliasing and poor
hinting.

It is better to default to "sans" and "monospace", which are mapped by
fontconfig to some appropriate font of the sysadmin and user's
choosing (typically Bitstream Vera Sans and Mono).  The result looks
more sensible and it makes gitk feel like a well-behaved software
citizen since its fonts match other native apps.

This patch does not change the appearance of gitk for users that have
already run it, since gitk uses the remembered UI and diff font names
from ~/.gitk.

Requested-by: Michael Biebl <biebl@debian.org>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Acked-by: Mark Hills <mark@pogo.org.uk>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>