git-gui: Display the "Loading annotation..." message in italic

git-gui: Display the "Loading annotation..." message in italic

If the user clicks on a line region that we haven't yet received
an annotation for from git-blame we show them "Loading annotation".
But I don't want the user to confuse this loading message with a
commit whose first line is "Loading annotation" and think we messed
up our display somehow.  Since we never use italics for anything
else, I'm going with the idea that italic slant can be used to show
data is missing/elided out at the time being.

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