Windows: fix utime() for read-only files

Starting with 5256b00 (Use git_mkstemp_mode instead of plain mkstemp to
create object files, 2010-02-22) utime() is invoked on read-only files.
This is not allowed on Windows and results in many warnings of the form

failed utime() on .git/objects/23/tmp_obj_VlgHlc: Permission denied

during a repack.  Fix it by making the file temporarily writable.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.c

@@ -259,8 +259,17 @@ int mingw_utime (const char *file_name, const struct utimbuf *times)
 	int fh, rc;
 
 	/* must have write permission */
-	if ((fh = open(file_name, O_RDWR | O_BINARY)) < 0)
-		return -1;
+	DWORD attrs = GetFileAttributes(file_name);
+	if (attrs != INVALID_FILE_ATTRIBUTES &&
+	    (attrs & FILE_ATTRIBUTE_READONLY)) {
+		/* ignore errors here; open() will report them */
+		SetFileAttributes(file_name, attrs & ~FILE_ATTRIBUTE_READONLY);
+	}
+
+	if ((fh = open(file_name, O_RDWR | O_BINARY)) < 0) {
+		rc = -1;
+		goto revert_attrs;
+	}
 
 	time_t_to_filetime(times->modtime, &mft);
 	time_t_to_filetime(times->actime, &aft);
@@ -270,6 +279,13 @@ int mingw_utime (const char *file_name, const struct utimbuf *times)
 	} else
 		rc = 0;
 	close(fh);
+
+revert_attrs:
+	if (attrs != INVALID_FILE_ATTRIBUTES &&
+	    (attrs & FILE_ATTRIBUTE_READONLY)) {
+		/* ignore errors again */
+		SetFileAttributes(file_name, attrs);
+	}
 	return rc;
 }