tcd: Determine correct scope from user input

If a user specifies both {window} and {tab}, `getcwd()`/`haslocaldir()`
are using "tab" as the scope that should be reported.  However, it
should be using "window" as the scope, within the specified tab page.

src/nvim/ex_docmd.h

@@ -26,6 +26,7 @@
 /// `getcwd()`. When using scopes as limits (e.g. in loops) don't use the scopes
 /// directly, use `MIN_CD_SCOPE` and `MAX_CD_SCOPE` instead.
 typedef enum {
+  kCdScopeInvalid = -1,
   kCdScopeWindow,  ///< Affects one window.
   kCdScopeTab,     ///< Affects one tab page.
   kCdScopeGlobal,  ///< Affects the entire instance of Neovim.