feat: implement fallback for some finders

This implements idea "2" from here:

- #393 (comment)

> We teach the callers to interpret an empty result list as a cue to
> try again using a fallback mechanism; in an empty directory this would
> give a false negative but a relatively harmless one (ie. you might run
> `:CommandTRipgrep` in an empty folder, and Command-T would take that as
> a cue to try again with the standard scanner, which would of course
> also return an empty list).

Specifically, we do this whenever `git`, `rg`, or `find` returns no
results.

There is a bit of duplication here because we need thunks at each site;
otherwise as soon as we `require`-d the fallback finder it would do a
redundant scan. I'm not going to factor that out yet because I am still
not convinced that this is an optimal strategy. Want to let it bake for
a while and see what happens.

lua/wincent/commandt/private/finders/find.lua

@@ -5,14 +5,23 @@ local ffi = require('ffi')
 
 -- TODO: remember cached directories
 return function(directory, options)
+  local finder = {}
+  -- Use a thunk to avoid cost of fallback scanning until actually needed.
+  finder.fallback = (
+    (function(d, o)
+      return function()
+        finder.fallback = require('wincent.commandt.private.finders.file')(d ~= '' and d or '.', o)
+        return finder.fallback
+      end
+    end)(directory, options)
+  )
   if vim.startswith(directory, './') then
     directory = directory:sub(3, -1)
   end
   if directory ~= '' and directory ~= '.' then
     directory = vim.fn.shellescape(directory)
   end
   local lib = require('wincent.commandt.private.lib')
-  local finder = {}
   finder.scanner = require('wincent.commandt.private.scanners.find').scanner(directory)
   finder.matcher = lib.matcher_new(finder.scanner, options)
   finder.run = function(query)

lua/wincent/commandt/private/finders/git.lua

@@ -7,11 +7,20 @@ local ffi = require('ffi')
 -- the `command` scanner
 -- TODO: remember cached directories
 return function(directory, options)
+  local finder = {}
+  -- Use a thunk to avoid cost of fallback scanning until actually needed.
+  finder.fallback = (
+    (function(d, o)
+      return function()
+        finder.fallback = require('wincent.commandt.private.finders.file')(d ~= '' and d or '.', o)
+        return finder.fallback
+      end
+    end)(directory, options)
+  )
   if directory ~= '' then
     directory = vim.fn.shellescape(directory)
   end
   local lib = require('wincent.commandt.private.lib')
-  local finder = {}
   finder.scanner = require('wincent.commandt.private.scanners.git').scanner(directory, options.scanners.git)
   finder.matcher = lib.matcher_new(finder.scanner, options)
   finder.run = function(query)

lua/wincent/commandt/private/finders/rg.lua

@@ -5,14 +5,23 @@ local ffi = require('ffi')
 
 -- TODO: remember cached directories
 return function(directory, options)
+  local finder = {}
+  -- Use a thunk to avoid cost of fallback scanning until actually needed.
+  finder.fallback = (
+    (function(d, o)
+      return function()
+        finder.fallback = require('wincent.commandt.private.finders.file')(d ~= '' and d or '.', o)
+        return finder.fallback
+      end
+    end)(directory, options)
+  )
   if vim.startswith(directory, './') then
     directory = directory:sub(3, -1)
   end
   if directory ~= '' and directory ~= '.' then
     directory = vim.fn.shellescape(directory)
   end
   local lib = require('wincent.commandt.private.lib')
-  local finder = {}
   finder.scanner = require('wincent.commandt.private.scanners.rg').scanner(directory)
   finder.matcher = lib.matcher_new(finder.scanner, options)
   finder.run = function(query)

lua/wincent/commandt/private/ui.lua

@@ -94,7 +94,14 @@ ui.show = function(finder, options)
     margin = options.margin,
     name = options.name,
     on_change = function(query)
-      results = finder.run(query)
+      results = current_finder.run(query)
+      if #results > 0 then
+        -- Once we've proved a finder works, we don't ever want to use fallback.
+        current_finder.fallback = nil
+      elseif current_finder.fallback then
+        current_finder = current_finder.fallback()
+        results = current_finder.run(query)
+      end
       if #results == 0 then
         selected = nil
       else