diff: allow unstuck arguments with --diff-algorithm

The argument to --diff-algorithm is mandatory, so there is no reason to
require the argument to be stuck to the option with '='.  Change this
for consistency with other Git commands.

Note that this does not change the handling of diff-algorithm in
merge-recursive.c since the primary interface to that is via the -X
option to 'git merge' where the unstuck form does not make sense.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -3634,15 +3634,16 @@ int diff_opt_parse(struct diff_options *options, const char **av, int ac)
 		options->xdl_opts = DIFF_WITH_ALG(options, PATIENCE_DIFF);
 	else if (!strcmp(arg, "--histogram"))
 		options->xdl_opts = DIFF_WITH_ALG(options, HISTOGRAM_DIFF);
-	else if (!prefixcmp(arg, "--diff-algorithm=")) {
-		long value = parse_algorithm_value(arg+17);
+	else if ((argcount = parse_long_opt("diff-algorithm", av, &optarg))) {
+		long value = parse_algorithm_value(optarg);
 		if (value < 0)
 			return error("option diff-algorithm accepts \"myers\", "
 				     "\"minimal\", \"patience\" and \"histogram\"");
 		/* clear out previous settings */
 		DIFF_XDL_CLR(options, NEED_MINIMAL);
 		options->xdl_opts &= ~XDF_DIFF_ALGORITHM_MASK;
 		options->xdl_opts |= value;
+		return argcount;
 	}
 
 	/* flags options */