pathencode: eliminate signed integer warnings

Compiling mercurial with the Sun Studio compiler gives seven copies of the
following warning on pathencode.c:

    line 533: warning: initializer will be sign-extended: -1

Using explicit unsigned literals silences it.

mercurial/pathencode.c

@@ -530,7 +530,7 b' static Py_ssize_t auxencode(char *dest, '
 	static const uint32_t twobytes[8];
 
 	static const uint32_t onebyte[8] = {
-		~0, 0xffff3ffe, ~0, ~0, ~0, ~0, ~0, ~0,
+		~0U, 0xffff3ffe, ~0U, ~0U, ~0U, ~0U, ~0U, ~0U,
 	};
 
 	return _encode(twobytes, onebyte, dest, 0, destsize, src, len, 0);