Set varstash_dir for "leave" event (#72)

Fixes https://github.com/Tarrasch/zsh-autoenv/issues/45.

tests/varstash-unstash-on-leave.t

@@ -0,0 +1,25 @@
+Test unstash behavior on leaving.
+
+  $ source $TESTDIR/setup.zsh || return 1
+
+Setup test environment.
+
+  $ mkdir sub
+  $ echo 'echo ENTER; stash FOO=changed' >| sub/$AUTOENV_FILE_ENTER
+  $ echo 'echo LEAVE; unstash FOO' >| sub/$AUTOENV_FILE_LEAVE
+  $ test_autoenv_auth_env_files sub
+  $ FOO=orig
+
+Activating the env stashes it and applies a new value.
+
+  $ cd sub
+  ENTER
+  $ echo $FOO
+  changed
+
+Leaving the directory unstashes it (varstash_dir is set to prev dir).
+
+  $ cd ..
+  LEAVE
+  $ echo $FOO
+  orig