Merge pull request #1800 from ethomson/mingw_warnings

Quiet down some warnings

src/win32/mingw-compat.h

@@ -21,7 +21,7 @@
 
 GIT_INLINE(size_t) p_strnlen(const char *s, size_t maxlen) {
 	const char *end = memchr(s, 0, maxlen);
-	return end ? (end - s) : maxlen;
+	return end ? (size_t)(end - s) : maxlen;
 }
 
 #endif