Merge branch 'dw/diff-no-index-doc'

When the user types "git diff" outside a working tree, thinking he
is inside one, the current error message that is a single-liner
"usage: git diff --no-index <path> <path>" may not be sufficient to
make him realize the mistake. Add "Not a git repository" to the
error message when we fell into the "--no-index" mode without an
explicit command line option to instruct us to do so.

* dw/diff-no-index-doc:
  diff --no-index: describe in a separate paragraph
  diff --no-index: clarify operation when not inside a repository

Documentation/git-diff.txt

@@ -28,10 +28,15 @@ two blob objects, or changes between two files on disk.
 	words, the differences are what you _could_ tell Git to
 	further add to the index but you still haven't.  You can
 	stage these changes by using linkgit:git-add[1].
-+
-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.
+
+'git diff' --no-index [--options] [--] [<path>...]::
+
+	This form is to compare the given two paths on the
+	filesystem.  You can omit the `--no-index` option when
+	running the command in a working tree controlled by Git and
+	at least one of the paths points outside the working tree,
+	or when running the command outside a working tree
+	controlled by Git.
 
 '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; ) {