diff: avoid some nesting

Avoid some nesting in builtin/diff.c, to make the code easier to read.
There are no functional changes.

Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/diff.c

@@ -319,27 +319,26 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 
 	init_revisions(&rev, prefix);
 
-	if (no_index) {
-		if (argc != i + 2) {
-			if (no_index == DIFF_NO_INDEX_IMPLICIT) {
-				/*
-				 * There was no --no-index and there were not two
-				 * paths. It is possible that the user intended
-				 * to do an inside-repository operation.
-				 */
-				fprintf(stderr, "Not a git repository\n");
-				fprintf(stderr,
-					"To compare two paths outside a working tree:\n");
-			}
-			/* Give the usage message for non-repository usage and exit. */
-			usagef("git diff %s <path> <path>",
-			       no_index == DIFF_NO_INDEX_EXPLICIT ?
-					"--no-index" : "[--no-index]");
-
+	if (no_index && argc != i + 2) {
+		if (no_index == DIFF_NO_INDEX_IMPLICIT) {
+			/*
+			 * There was no --no-index and there were not two
+			 * paths. It is possible that the user intended
+			 * to do an inside-repository operation.
+			 */
+			fprintf(stderr, "Not a git repository\n");
+			fprintf(stderr,
+				"To compare two paths outside a working tree:\n");
 		}
+		/* Give the usage message for non-repository usage and exit. */
+		usagef("git diff %s <path> <path>",
+		       no_index == DIFF_NO_INDEX_EXPLICIT ?
+		       "--no-index" : "[--no-index]");
+
+	}
+	if (no_index)
 		/* If this is a no-index diff, just run it and exit there. */
 		diff_no_index(&rev, argc, argv, prefix);
-	}
 
 	/* Otherwise, we are doing the usual "git" diff */
 	rev.diffopt.skip_stat_unmatch = !!diff_auto_refresh_index;