fix(path): make find_upwards() search root

The previous implementation of `Path:find_upwards()` used the loop
condition `folder ~= root` to bound the search which meant the root
directory would never be searched. This commit instead breaks the `while
true` loop after searching the root directory.

The function now also returns `nil` rather than a blank string to
indicate that the search has failed.

Finally, this commit adds specs for the function.

Author: shreve

lua/plenary/path.lua

@@ -931,11 +931,14 @@ function Path:find_upwards(filename)
   local folder = Path:new(self)
   local root = path.root(folder:absolute())
 
-  while folder:absolute() ~= root do
+  while true do
     local p = folder:joinpath(filename)
     if p:exists() then
       return p
     end
+    if folder:absolute() == root then
+      break
+    end
     folder = folder:parent()
   end
   return nil

tests/plenary/path_spec.lua

@@ -721,6 +721,34 @@ SOFTWARE.]]
       assert.are.same(should, data)
     end)
   end)
+
+  describe(":find_upwards", function()
+    it("finds files that exist", function()
+      local p = Path:new(debug.getinfo(1, "S").source:sub(2))
+      local found = p:find_upwards "README.md"
+      assert.are.same(found:absolute(), Path:new("README.md"):absolute())
+    end)
+
+    it("finds files that exist at the root", function()
+      local p = Path:new(debug.getinfo(1, "S").source:sub(2))
+
+      -- Temporarily set path.root to the root of this repository
+      local root = p.path.root
+      p.path.root = function(_)
+        return p:parent():parent():parent().filename
+      end
+
+      local found = p:find_upwards "README.md"
+      assert.are.same(found:absolute(), Path:new("README.md"):absolute())
+      p.path.root = root
+    end)
+
+    it("returns nil if no file is found", function()
+      local p = Path:new(debug.getinfo(1, "S").source:sub(2))
+      local found = p:find_upwards "MISSINGNO.md"
+      assert.are.same(found, nil)
+    end)
+  end)
 end)
 
 -- function TestPath:testIsDir()