merge-recursive: fix parsing of "diff-algorithm" option

The "diff-algorithm" option to the recursive merge strategy takes the
name of the algorithm as an option, but it uses strcmp on the option
string to check if it starts with "diff-algorithm=", meaning that this
options cannot actually be used.

Fix this by switching to prefixcmp.  At the same time, clarify the
following line by using strlen instead of a hard-coded length, which
also makes it consistent with nearby code.
Reported-by: NLuke Noel-Storr <luke.noel-storr@integrate.co.uk>
Signed-off-by: NJohn Keeping <john@keeping.me.uk>
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>

merge-recursive.c

@@ -2068,8 +2068,8 @@ int parse_merge_opt(struct merge_options *o, const char *s)
 		o->xdl_opts = DIFF_WITH_ALG(o, PATIENCE_DIFF);
 	else if (!strcmp(s, "histogram"))
 		o->xdl_opts = DIFF_WITH_ALG(o, HISTOGRAM_DIFF);
-	else if (!strcmp(s, "diff-algorithm=")) {
-		long value = parse_algorithm_value(s+15);
+	else if (!prefixcmp(s, "diff-algorithm=")) {
+		long value = parse_algorithm_value(s + strlen("diff-algorithm="));
 		if (value < 0)
 			return -1;
 		/* clear out previous settings */