Fix propagation of executable bit & flawed test

src commit has a special case to handle user-initiated change of executable bit on a tracked file since the (RCS) back-end will not respect such changes (it only respects executable bit at initial check-in). Unfortunately, this special case is broken and thus utterly non-functional. This patch fixes it.

It also fixes the flawed check of this functionality in srctest which failed to detect the broken behavior.