check stderr with isatty() instead of stdout when deciding to show progress

Make transport code (viz. transport.c::fetch_refs_via_pack() and
transport-helper.c::standard_options()) that decides to show progress
check if stderr is a terminal, instead of stdout. After all, progress
reports (via the API in progress.[ch]) are sent to stderr.

Update the documentation for git-clone to say "standard error" as well.

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-clone.txt

@@ -101,7 +101,7 @@ objects from the source repository into a pack in the cloned repository.
 
 --verbose::
 -v::
-	Display the progress bar, even in case the standard output is not
+	Display the progress bar, even in case the standard error is not
 	a terminal.
 
 --no-checkout::

transport-helper.c

@@ -144,7 +144,7 @@ static void standard_options(struct transport *t)
 	char buf[16];
 	int n;
 	int v = t->verbose;
-	int no_progress = v < 0 || (!t->progress && !isatty(1));
+	int no_progress = v < 0 || (!t->progress && !isatty(2));
 
 	set_helper_option(t, "progress", !no_progress ? "true" : "false");
 

transport.c

@@ -476,7 +476,7 @@ static int fetch_refs_via_pack(struct transport *transport,
 	args.include_tag = data->followtags;
 	args.verbose = (transport->verbose > 0);
 	args.quiet = (transport->verbose < 0);
-	args.no_progress = args.quiet || (!transport->progress && !isatty(1));
+	args.no_progress = args.quiet || (!transport->progress && !isatty(2));
 	args.depth = data->depth;
 
 	for (i = 0; i < nr_heads; i++)

transport.h

@@ -26,7 +26,7 @@ struct transport {
 	int (*disconnect)(struct transport *connection);
 	char *pack_lockfile;
 	signed verbose : 3;
-	/* Force progress even if the output is not a tty */
+	/* Force progress even if stderr is not a tty */
 	unsigned progress : 1;
 };