git-gui: Only allow Refresh in diff context menu when we have a diff.

There is no reason to attempt refreshing an empty diff viewer, so
the Refresh option of our diff context menu should be disabled when
there is no diff currently shown.

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

git-gui.sh

@@ -4261,6 +4261,7 @@ $ctxm add command \
 	-label {Refresh} \
 	-font font_ui \
 	-command reshow_diff
+lappend diff_actions [list $ctxm entryconf [$ctxm index last] -state]
 $ctxm add command \
 	-label {Copy} \
 	-font font_ui \