git-diff A...B to (usually) mean "git-diff `git-merge-base A B` B"

This tweaks the argument parser of "git diff" to allow "git-diff
A...B" to show diffs leading to B since their merge-base, when
there is only one sensible merge base between A and B.

Currently nonsense cases are thrown at combined-diff to produce
nonsense results, which would eventually need to be fixed.

Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-diff.c

@@ -346,7 +346,15 @@ int cmd_diff(int argc, const char **argv, char **envp)
 		return builtin_diff_index(&rev, argc, argv);
 	else if (ents == 2)
 		return builtin_diff_tree(&rev, argc, argv, ent);
+	else if ((ents == 3) && (ent[0].item->flags & UNINTERESTING)) {
+		/* diff A...B where there is one sane merge base between
+		 * A and B.  We have ent[0] == merge-base, ent[1] == A,
+		 * and ent[2] == B.  Show diff between the base and B.
+		 */
+		return builtin_diff_tree(&rev, argc, argv, ent);
+	}
 	else
-		return builtin_diff_combined(&rev, argc, argv, ent, ents);
+		return builtin_diff_combined(&rev, argc, argv,
+					     ent, ents);
 	usage(builtin_diff_usage);
 }