Fix zsh completion not working on the first time in a shell session (#1237)

The zsh completion script output by cobra is a stub completion function
which replaces itself with the actual completion function. This
technique enables cobra to define helper functions without splitting the
completion script into multiple files.  However, the current
implementation forgets to call the actual completion function at the end
of the stub function, meaning that completion won't work the first time
it's invoked in a shell session. This commit is a fix for this problem.

zsh_completions.go

@@ -229,6 +229,11 @@ _%[1]s()
         _describe "completions" completions $(echo $flagPrefix)
     fi
 }
+
+# don't run the completion function when being source-ed or eval-ed
+if [ "$funcstack[1]" = "_%[1]s" ]; then
+	_%[1]s
+fi
 `, name, compCmd,
 		ShellCompDirectiveError, ShellCompDirectiveNoSpace, ShellCompDirectiveNoFileComp,
 		ShellCompDirectiveFilterFileExt, ShellCompDirectiveFilterDirs))