feat: show "fallback" in prompt title if, er, falling back

As discussed here:

- #393 (comment)

That comment suggested being a bit more verbose:

    CommandT [ find (fall-backed from git) ]

but here I'm going with something a little less descriptive:

    CommandT [fallback]

seeing as the extra detail in the title doesn't tell you _what_ went
wrong; really, all we can do here is show a hint that _something_ went
wrong, so we might as well be brief about it.

Two things to note about implementation:

- One is that I am playing around here trying to figure out how much
  metatable magic I want in here without making it all too "clever". So
  here I have a `__newindex` implementation that allows us to write
  `prompt.name = 'fallback'` instead of `prompt:set_name('fallback')`.
  The former is visually cleaner, but I don't necessarily like that it
  hides the fact that an imperative side-effect is going to take place.
  I will probably "dumb this down" as opposed to trying to use the
  pattern in other places in a uniform way.

- The other is that my earlier claims about top-down React-style
  dataflow are pretty much not true any more. It _was_ true at the
  start, when each component rendered using data passed into it; now we
  have muddied the waters considerably with imperative methods. I will
  almost certainly want to revisit this topic in the future.

lua/wincent/commandt/private/finders/fallback.lua

@@ -6,6 +6,6 @@
 return function(finder, directory, options)
   return function()
     finder.fallback = require('wincent.commandt.private.finders.file')(directory ~= '' and directory or '.', options)
-    return finder.fallback
+    return finder.fallback, 'fallback'
   end
 end

lua/wincent/commandt/private/prompt.lua

@@ -10,6 +10,13 @@ local Prompt = {}
 
 local mt = {
   __index = Prompt,
+  __newindex = function(t, k, v)
+    if k == 'name' then
+      Prompt.set_name(t, v)
+    else
+      rawset(t, k, v)
+    end
+  end,
 }
 
 function Prompt.new(options)
@@ -47,6 +54,17 @@ function Prompt:close()
   end
 end
 
+function Prompt:set_name(name)
+  self._name = name
+  if self._window then
+    self._window:set_title(self:title())
+  end
+end
+
+function Prompt:title()
+  return self._name and ('CommandT [' .. self._name .. ']') or 'CommandT'
+end
+
 function Prompt:show()
   local bottom = nil
   local top = nil
@@ -64,7 +82,6 @@ function Prompt:show()
   end
 
   if self._window == nil then
-    local title = self._name and ('CommandT [' .. self._name .. ']') or 'CommandT'
     self._window = Window.new({
       bottom = bottom,
       buftype = 'prompt',
@@ -79,7 +96,7 @@ function Prompt:show()
         self._window = nil
       end,
       on_leave = self._on_leave,
-      title = title,
+      title = self:title(),
       top = top,
     })
   end

lua/wincent/commandt/private/ui.lua

@@ -99,7 +99,8 @@ ui.show = function(finder, options)
         -- 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()
+        current_finder, name = current_finder.fallback()
+        prompt.name = name or 'fallback'
         results = current_finder.run(query)
       end
       if #results == 0 then

lua/wincent/commandt/private/window.lua

@@ -198,6 +198,19 @@ function Window:replace_lines(lines, options)
   end
 end
 
+function Window:set_title(title)
+  self._title = title
+  self._padded_title = title ~= '' and (' ' .. title .. ' ') or ''
+  self:_reposition()
+  vim.api.nvim_buf_set_lines(
+    self._title_buffer,
+    0, -- start
+    -1, -- end
+    false, -- strict indexing
+    { self._padded_title }
+  )
+end
+
 function Window:show()
   if self._main_buffer == nil then
     self._main_buffer = vim.api.nvim_create_buf(
@@ -407,7 +420,7 @@ function Window:_reposition()
         col = position.col + #self._prompt,
         height = 1,
         row = math.max(0, position.row),
-        width = #(' ' .. self._title .. ' '),
+        width = #self._padded_title,
       })
     )
   end