Cache the current filepath when cwd == .

When editing a file in the current working directory, cache the filepath
just like when outside the cwd. (If b:Powerline_filepath isn't set, the
filepath isn't cached and vim recalculate everytime it draws the
statusline.)

Still doesn't cache when the filepath is empty.

autoload/Powerline/Functions.vim

@@ -36,7 +36,7 @@ function! Powerline#Functions#GetFilepath() " {{{
 	endif
 
 	if ret == ('.' . dirsep)
-		return ''
+		let ret = ''
 	endif
 
 	let b:Powerline_filepath = ret