dgit: unrepres. changes: Tolerate creation of symlinks

dgit: unrepres. changes: Tolerate creation of symlinks

Experimentally, dpkg-source on stretch will create patches to create
new symlinks.  (It will fail on attempts to modify existing symlinks
and it ignores attempts to change plain file executability.)

Implementation: add an alternative which tolerates the git symlink
mode.  This replaces the check on $oldmode, which in this context we
know is all 0s and can therefore never match.

While we're here, change the error message.

Closes:#857382.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>