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/diff-index.c

@@ -7,7 +7,7 @@
 
 static const char diff_cache_usage[] =
 "git diff-index [-m] [--cached] "
-"[<common diff options>] <tree-ish> [<path>...]"
+"[<common-diff-options>] <tree-ish> [<path>...]"
 COMMON_DIFF_OPTIONS_HELP;
 
 int cmd_diff_index(int argc, const char **argv, const char *prefix)