pack-object: squelch eye-candy on non-tty

One of my post-update scripts runs a git-fetch into a separate
repository and sends the results back to me (2>&1); I end up
getting this in the mail:

    Generating pack...
    Done counting 180 objects.
    Result has 131 objects.
    Deltifying 131 objects.
       0% (0/131) done^M   1% (2/131) done^M...

This defaults not to do the progress report when not on a tty.

You could give --progress to force the progress report, but
let's not bother even documenting it nor mentioning it in the
usage string.

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

pack-objects.c

@@ -1239,6 +1239,7 @@ int main(int argc, char **argv)
 
 	setup_git_directory();
 
+	progress = isatty(2);
 	for (i = 1; i < argc; i++) {
 		const char *arg = argv[i];
 
@@ -1269,6 +1270,10 @@ int main(int argc, char **argv)
 					usage(pack_usage);
 				continue;
 			}
+			if (!strcmp("--progress", arg)) {
+				progress = 1;
+				continue;
+			}
 			if (!strcmp("-q", arg)) {
 				progress = 0;
 				continue;