diff --no-index: clarify operation when not inside a repository

Clarify documentation for "diff --no-index".  State that when not
inside a repository, --no-index is implied and two arguments are
mandatory.

Clarify error message from diff-no-index to inform user that CWD is
not inside a repository and thus two arguments are mandatory.

Signed-off-by: Dale Worley <worley@ariadne.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: daleworley

Documentation/git-diff.txt

@@ -31,7 +31,8 @@ two blob objects, or changes between two files on disk.
 +
 If exactly two paths are given and at least one points outside
 the current repository, 'git diff' will compare the two files /
-directories. This behavior can be forced by --no-index.
+directories. This behavior can be forced by --no-index or by
+executing 'git diff' outside of a working tree.
 
 'git diff' [--options] --cached [<commit>] [--] [<path>...]::
 

diff-no-index.c

@@ -215,9 +215,21 @@ void diff_no_index(struct rev_info *revs,
 		     path_inside_repo(prefix, argv[i+1])))
 			return;
 	}
-	if (argc != i + 2)
+	if (argc != i + 2) {
+		if (!no_index) {
+			/*
+			 * 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 ? "--no-index" : "[--no-index]");
+	}
 
 	diff_setup(&revs->diffopt);
 	for (i = 1; i < argc - 2; ) {