Improve path normalisation with separate substitutions

This change reverts the original regex and simply adds a second regex
which is called on the result. '/\./' is one literal forward slash, one
literal fullstop (escaped with a leading backslash), and one literal
forward slash. In plain text, `/./` will be replaced with `/`.

autoload/vimwiki/path.vim

@@ -24,7 +24,8 @@ endif
 function! vimwiki#path#normalize(path) abort
   let path = a:path
   while 1
-    let result = substitute(path, '/[^/]\+/\.\.\|\./', '', '')
+    let intermediateResult = substitute(path, '/[^/]\+/\.\.', '', '')
+    let result = substitute(intermediateResult, '/\./', '/', '')
     if result ==# path
       break
     endif