pickaxe: call strlen only when necessary in diffcore_pickaxe_count()

We need to determine the search term's length only when fixed-string
matching is used; regular expression compilation takes a NUL-terminated
string directly.  Only call strlen() in the former case.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diffcore-pickaxe.c

@@ -205,7 +205,6 @@ void diffcore_pickaxe(struct diff_options *o)
 {
 	const char *needle = o->pickaxe;
 	int opts = o->pickaxe_opts;
-	unsigned long len = strlen(needle);
 	regex_t regex, *regexp = NULL;
 	kwset_t kws = NULL;
 
@@ -226,7 +225,7 @@ void diffcore_pickaxe(struct diff_options *o)
 	} else {
 		kws = kwsalloc(DIFF_OPT_TST(o, PICKAXE_IGNORE_CASE)
 			       ? tolower_trans_tbl : NULL);
-		kwsincr(kws, needle, len);
+		kwsincr(kws, needle, strlen(needle));
 		kwsprep(kws);
 	}