Fix the reference character check for Unicode

We need to do an unsigned comparison, as otherwise UTF-8 characters
might look like they have the sign bit set and the check will fail.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>

src/refs.c

@@ -1647,7 +1647,7 @@ void git_repository__refcache_free(git_refcache *refs)
  *****************************************/
 static int check_valid_ref_char(char ch)
 {
-	if (ch <= ' ')
+	if ((unsigned) ch <= ' ')
 		return GIT_ERROR;
 
 	switch (ch) {
@@ -1752,4 +1752,4 @@ int git_reference__normalize_name(char *buffer_out, size_t out_size, const char
 int git_reference__normalize_name_oid(char *buffer_out, size_t out_size, const char *name)
 {
 	return normalize_name(buffer_out, out_size, name, 1);
-}
\ No newline at end of file
+}