standardize usage info string format

This patch puts the usage info strings that were not already in docopt-
like format into docopt-like format, which will be a litle easier for
end users and a lot easier for translators. Changes include:

- Placing angle brackets around fill-in-the-blank parameters
- Putting dashes in multiword parameter names
- Adding spaces to [-f|--foobar] to make [-f | --foobar]
- Replacing <foobar>* with [<foobar>...]

Signed-off-by: Alex Henrie <alexhenrie24@gmail.com>
Reviewed-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge-index.c

@@ -75,7 +75,7 @@ int cmd_merge_index(int argc, const char **argv, const char *prefix)
 	signal(SIGCHLD, SIG_DFL);
 
 	if (argc < 3)
-		usage("git merge-index [-o] [-q] <merge-program> (-a | [--] <filename>*)");
+		usage("git merge-index [-o] [-q] <merge-program> (-a | [--] [<filename>...])");
 
 	read_cache();