Only source varstash lib if it's being used

I have noticed that `autostash` called an (accidentally)
defined/overwritten `stash` function.

While it's not possible to put this into a local scope, this will at
least make it less polluting by default (when autostashing is not used).

Additionally we could source it always (not just once), but then it
would only re-overwrite the function.

A better fix would be to prefix the functions explicitly, e.g.
`autoenv_autostash` etc.

Closes https://github.com/Tarrasch/zsh-autoenv/pull/42.

autoenv.zsh

@@ -260,8 +260,12 @@ _autoenv_source() {
   local autoenv_to_dir=$PWD
 
   # Source varstash library once.
+  # XXX: pollutes environment with e.g. `stash`, and `autostash` will cause
+  # an overwritten `stash` function to be called!
   if [[ -z "$functions[(I)autostash]" ]]; then
+    if \grep -qE '\b(autostash|autounstash|stash)\b' $autoenv_env_file; then
       source ${${funcsourcetrace[1]%:*}:h}/lib/varstash
+    fi
     # NOTE: Varstash uses $PWD as default for varstash_dir, we might set it to
     # ${autoenv_env_file:h}.
   fi
@@ -331,7 +335,9 @@ _autoenv_chpwd_handler() {
         fi
 
         # Unstash any autostashed stuff.
+        if [[ -n "$functions[(I)autostash]" ]]; then
           varstash_dir=$prev_dir autounstash
+        fi
 
         _autoenv_stack_entered_remove $prev_file
       fi

tests/varstash.t

@@ -6,11 +6,22 @@ Setup test environment.
 
   $ mkdir sub
   $ cd sub
+
+The varstash library should not get loaded always.
+
+  $ echo 'echo ENTER' > $AUTOENV_FILE_ENTER
+  $ echo 'echo LEAVE' > $AUTOENV_FILE_LEAVE
+  $ test_autoenv_auth_env_files
+  $ cd .
+  ENTER
+  $ type -w autostash
+  autostash: none
+  [1]
+
+Now on to some stashing.
+
   $ echo 'echo ENTER; autostash FOO=changed' > $AUTOENV_FILE_ENTER
   $ echo 'echo LEAVE; autounstash' > $AUTOENV_FILE_LEAVE
-
-Manually create auth file
-
   $ test_autoenv_auth_env_files
 
 Set environment variable.
@@ -19,8 +30,12 @@ Set environment variable.
 
 Activating the env stashes it and applies a new value.
 
-  $ cd .
+  $ cd ..
+  LEAVE
+  $ cd sub
   ENTER
+  $ type -w autostash
+  autostash: function
   $ echo $FOO
   changed