Executing the script 'tst' (included in the patch)
on a window clears its contents, but the change
can't be undone.  This patch fixes it.

Also, I think xfid.c:218 should be removed because
xfid.c:224 takes care of resetting 'nomark' after
all open Xfids of data file are closed, but I
haven't tested it — hence not included in this
patch.