git-gui: Highlight the blame commit header from everything else
git-gui: Highlight the blame commit header from everything else

The selected commit's blame header is now drawn in green, using
the same background color that is shown in the main file content
viewer.  The result is a much better looking commit pane, as we
use bold for header "keys" and proportional width fonts for the
stuff that doesn't need to be fixed width to maintain its formatting.

Signed-off-by: Shawn O. Pearce <f01032651c12a22b2103346ba96b3b6b231f9d3c@spearce.org>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git