use reletive paths for edit commands if possible

plugin/NERD_tree.vim

@@ -1539,12 +1539,21 @@ endfunction
 "Return: the string for this path that is suitable to be used with the :edit
 "command
 function! s:Path.strForEditCmd()
+    let p = self.str(1)
+    let cwd = getcwd()
+
     if s:running_windows
-        return self.strForOS(0)
-    else
-        return self.str(1)
+        let p = tolower(self.strForOS(0))
+        let cwd = tolower(getcwd())
     endif
 
+    "return a relative path if we can
+    if stridx(p, cwd) == 0
+        let p = strpart(p, strlen(cwd)+1)
+    endif
+
+    return p
+
 endfunction
 "FUNCTION: Path.strForGlob() {{{3
 function! s:Path.strForGlob()