Fix pass zsh completion and autoloading

When autocompleting from `pass <TAB>', sometimes the following errors
appear:

  _values:compvalues:10: not enough arguments
  find: `/home/user/.password-store': No such file or directory
  _values:compvalues:10: not enough arguments
  find: `/home/user/.password-store': No such file or directory

The `_values' error happens when there is no password-store folder *or*
there are no passwords in pass; the `find' error only when there is no
password-store folder.

We can trace it back to line 108, which contains the only `_values'
statement that is executed when we autocomplete from pass. We confirm
this by following the trail of execution, which is

  _pass -> _pass_cmd_show -> _pass_complete_entries ->
        -> _pass_complete_entries_helper

If we try running the command inside `$()' on line 104, we see that it
returns nothing and the output is blank. This means that `_values' only
receives 1 of its 2 mandatory parameters, therefore the above error is
triggered (not enough arguments).

That is unless we don't have a password-store folder, in which case the
`find: [...] no such file or directory' error is *also* triggered.

We solve the first error by supplying a default value of "" if the
command outputs nothing, using the zsh construct ${var:-else}.

We solve the second error by redirecting the find command's stderr output
to /dev/null, so the error is effectively suppressed.

* * * *

This patch also fixes the first tab completion, which currently only
loads the completion function definition.

We do this by adding a `_pass' statement at the end of the file, which
runs the `_pass' completion function after loading its definition.
This is the standard way an autoloaded function works; for other examples
look at zsh's official completion files.

plugins/pass/_pass

@@ -118,7 +118,7 @@ _pass_cmd_show () {
 _pass_complete_entries_helper () {
 	local IFS=$'\n'
 	local prefix="${PASSWORD_STORE_DIR:-$HOME/.password-store}"
-	_values -C 'passwords' $(find -L "$prefix" \( -name .git -o -name .gpg-id \) -prune -o $@ -print | sed -e "s#${prefix}/\{0,1\}##" -e 's#\.gpg##' | sort)
+	_values -C 'passwords' ${$(find -L "$prefix" \( -name .git -o -name .gpg-id \) -prune -o $@ -print 2>/dev/null | sed -e "s#${prefix}/\{0,1\}##" -e 's#\.gpg##' | sort):-""}
 }
 
 _pass_complete_entries_with_subdirs () {
@@ -134,3 +134,5 @@ _pass_complete_keys () {
 	# Extract names and email addresses from gpg --list-keys
 	_values 'gpg keys' $(gpg2 --list-secret-keys --with-colons | cut -d : -f 10 | sort -u | sed '/^$/d')
 }
+
+_pass