parse-opts: prepare for OPT_FILENAME

To give OPT_FILENAME the prefix, we pass the prefix to parse_options()
which passes the prefix to parse_options_start() which sets the prefix
member of parse_opts_ctx accordingly. If there isn't a prefix in the
calling context, passing NULL will suffice.

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-parse-options.txt

@@ -60,13 +60,13 @@ Steps to parse options
 . in `cmd_foo(int argc, const char **argv, const char *prefix)`
   call
 
-	argc = parse_options(argc, argv, builtin_foo_options, builtin_foo_usage, flags);
+	argc = parse_options(argc, argv, prefix, builtin_foo_options, builtin_foo_usage, flags);
 +
 `parse_options()` will filter out the processed options of `argv[]` and leave the
 non-option arguments in `argv[]`.
 `argc` is updated appropriately because of the assignment.
 +
-You can also pass NULL instead of a usage array as fourth parameter of
+You can also pass NULL instead of a usage array as the fifth parameter of
 parse_options(), to avoid displaying a help screen with usage info and
 option list.  This should only be done if necessary, e.g. to implement
 a limited parser for only a subset of the options that needs to be run