git-gui: Changed blame header bar background to match main window

git-gui: Changed blame header bar background to match main window

The main window's diff header bar background switched from orange
to gold recently, and I liked the effect it had on readability of
the text.  Since I wanted the blame viewer to match, here it is.

Though this probably should be a user defined color, or at least
a constant somewhere that everyone can reference.

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