mingw: do not call xutftowcs_path in mingw_mktemp

The `xutftowcs_path` function canonicalizes absolute paths using GetFullPathNameW.
This canonicalization may change the length of the string (e.g. getting rid of \.\),
which breaks callers that pass the template string in a strbuf and expect the
length of the string to remain the same.

In my particular case, the tmp-objdir code is passing a strbuf to mkdtemp and is
breaking since the strbuf.len is no longer synchronized with strlen(strbuf.buf).

Signed-off-by: Neeraj K. Singh <neerajsi@microsoft.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.c

@@ -1235,8 +1235,11 @@ char *mingw_mktemp(char *template)
 	int offset = 0;
 
 	/* we need to return the path, thus no long paths here! */
-	if (xutftowcs_path(wtemplate, template) < 0)
+	if (xutftowcsn(wtemplate, template, MAX_PATH, -1) < 0) {
+		if (errno == ERANGE)
+			errno = ENAMETOOLONG;
 		return NULL;
+	}
 
 	if (is_dir_sep(template[0]) && !is_dir_sep(template[1]) &&
 	    iswalpha(wtemplate[0]) && wtemplate[1] == L':') {