ui-diff: add "stat only" diff type

This prints the diffstat but stops before printing (or generating) any
of the body of the diff.

No cgitrc option is added here so that we can wait to see how useful
this is before letting people set it as the default.

Suggested-by: Konstantin Ryabitsev <mricon@kernel.org>
Signed-off-by: John Keeping <john@keeping.me.uk>

cgit.h

@@ -53,7 +53,7 @@ typedef void (*filepair_fn)(struct diff_filepair *pair);
 typedef void (*linediff_fn)(char *line, int len);
 
 typedef enum {
-	DIFF_UNIFIED, DIFF_SSDIFF
+	DIFF_UNIFIED, DIFF_SSDIFF, DIFF_STATONLY
 } diff_type;
 
 typedef enum {