Fix Path:find_upwards()

The prior implementation had several implementation issues which caused
an infinite loop if the search turned up empty.

This new implementation corrects the bounds check to stop searching once
we've searched everywhere we can, and properly searches the root
directory

Author: shreve

lua/plenary/path.lua

@@ -296,7 +296,11 @@ function Path:_st_mode()
 end
 
 function Path:joinpath(...)
-  return Path:new(self.filename, ...)
+  if string.char(self.filename:byte(-1)) == path.sep then
+    return Path:new(self.filename:sub(0, -2), ...)
+  else
+    return Path:new(self.filename, ...)
+  end
 end
 
 function Path:absolute()
@@ -917,14 +921,20 @@ end
 
 function Path:find_upwards(filename)
   local folder = Path:new(self)
-  while self:absolute() ~= path.root do
+  local root = path.root(folder:absolute())
+
+  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 ""
+
+  return nil
 end
 
 return Path