Add strchrnul()

As suggested by Pierre Habouzit, add strchrnul().  It's a useful GNU
extension and can simplify string parser code.  There are several
places in git that can be converted to strchrnul(); as a trivial
example, this patch introduces its usage to builtin-fetch--tool.c.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -183,6 +183,11 @@ void *gitmemmem(const void *haystack, size_t haystacklen,
                 const void *needle, size_t needlelen);
 #endif
 
+#ifdef NO_STRCHRNUL
+#define strchrnul gitstrchrnul
+char *gitstrchrnul(const char *s, int c);
+#endif
+
 extern void release_pack_memory(size_t, int);
 
 static inline char* xstrdup(const char *str)