wrapper: add function to compare strings with different NUL termination

When parsing capabilities for the pack protocol, there are times we'll
want to compare the value of a capability to a NUL-terminated string.
Since the data we're reading will be space-terminated, not
NUL-terminated, we need a function that compares the two strings, but
also checks that they're the same length.  Otherwise, if we used strncmp
to compare these strings, we might accidentally accept a parameter that
was a prefix of the expected value.

Add a function, xstrncmpz, that takes a NUL-terminated string and a
non-NUL-terminated string, plus a length, and compares them, ensuring
that they are the same length.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

wrapper.c

@@ -105,6 +105,14 @@ char *xstrndup(const char *str, size_t len)
 	return xmemdupz(str, p ? p - str : len);
 }
 
+int xstrncmpz(const char *s, const char *t, size_t len)
+{
+	int res = strncmp(s, t, len);
+	if (res)
+		return res;
+	return s[len] == '\0' ? 0 : 1;
+}
+
 void *xrealloc(void *ptr, size_t size)
 {
 	void *ret;