dgit: unrepres. changes: Prepare to tolerate symlinks

dgit: unrepres. changes: Prepare to tolerate symlinks

Permit symlinks (which have mode 120000 in git) to make their way
through for more detailed checks.

No functional change except to error messages, because:

 * If neither thing was a symlink, then the existing regexps
   still match and the new "modified symlink" clause will not,
   so the flow is unchanged.

 * Otherwise, if both $oldmode and $newmode match [^0], ie,
   this is a modification rather than deletion or removal,
   we insist that $oldmode=$newmode, and then, fail the
   new "modified symlink" check.

 * Otherwise, we fail the check for default mode.

So in the case we are allowing to proceed further, we fail as before.

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