git-gui: Refactor diff popup into a procedure to ease i18n work
git-gui: Refactor diff popup into a procedure to ease i18n work

The folks working on the i18n version of git-gui have had some
trouble trying to convert these English strings into [mc] calls
due to the double evaluation.  Moving this block into a standard
procedure eliminates the double evaluation, making their work
easier.

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