Always use "." for cd

Nobody doesn't want to use $PWD to cd, so if $CDPATH does not include
it that was a mistake.

Bash also appends "." here.

Fixes #4484.

tests/cd.out

@@ -23,3 +23,9 @@ cd:
 PWD is /tmp/cdcomp_test/linkhome
 BogusPWD test 1 succeeded
 BogusPWD test 2 succeeded
+
+####################
+# CDPATH
+Gone to linkhome via CDPATH
+Gone to base
+Gone to linkhome via implicit . in CDPATH