Use empty CDPATH with cd in abs_dirname/expand_path

While not recommended [1] `CDPATH` might be exported and would be used
for e.g. `cd test` instead of using `./test`.
E.g. Vim will make use of it (and therefore it needs to be exported, if
you do not want to configure it twice).

Fixes #104.

This supersedes #119: there is
no need to unset/change CDPATH globally.

1: https://bosker.wordpress.com/2012/02/12/bash-scripters-beware-of-the-cdpath/

libexec/bats

@@ -35,19 +35,19 @@ abs_dirname() {
   local path="$1"
 
   while [ -n "$path" ]; do
-    cd "${path%/*}"
+    CDPATH= cd "${path%/*}"
     local name="${path##*/}"
     path="$(resolve_link "$name" || true)"
   done
 
   pwd
-  cd "$cwd"
+  CDPATH= cd "$cwd"
 }
 
 expand_path() {
-  { cd "$(dirname "$1")" 2>/dev/null
+  { CDPATH= cd "$(dirname "$1")" 2>/dev/null
     local dirname="$PWD"
-    cd "$OLDPWD"
+    CDPATH= cd "$OLDPWD"
     echo "$dirname/$(basename "$1")"
   } || echo "$1"
 }