git-gui: if a background colour is set, set foreground colour as well

git-gui: if a background colour is set, set foreground colour as well

In several places, only the background colour is set to an explicit
value, sometimes even "white".  This does not work well with dark
colour themes.

This patch tries to set the foreground colour to "black" in those
situations, where an explicit background colour is set without defining
any foreground colour.

Signed-off-by: Philipp A. Hartmann <redacted>
Signed-off-by: Shawn O. Pearce <redacted>