Fix Renaming issue in Windows and ext with two dot (#1051)

* fix subdir not correct when path using `\`

* fix slice by variable value not supported in vim 7.3

autoload/vimwiki/base.vim

@@ -62,12 +62,19 @@ function! vimwiki#base#subdir(path, filename) abort
     let filename = a:filename
   endif
   let idx = 0
+  let pathelement = split(path, '[/\\]') 
+  let fileelement = split(filename, '[/\\]')
+  let minlen = min([len(pathelement), len(fileelement)])
+  let p = fileelement[:]
   "FIXME this can terminate in the middle of a path component!
-  while path[idx] ==? filename[idx]
+  while pathelement[idx] ==? fileelement[idx]
+    let p = p[1:]
     let idx = idx + 1
+    if idx == minlen
+      break
+    endif
   endwhile
 
-  let p = split(strpart(filename, idx), '[/\\]')
   let res = join(p[:-2], '/')
   if len(res) > 0
     let res = res.'/'