Don't double up on directory trailing slash

[Dominic Martinez]

Directory replacements always add a trailing slash, even if the directory string has one:

D=directory/
-
directory//foo
directory//bar.a
directory//foobar.bee
directory//foo
directory//foo.a
directory//bar.bee
directory//foobar.so
directory/
<grist>directory//bar.bee(archive.o)

This should be a compatibility behavior, but off by default.