vim-patch:8.2.0985: simplify() does not remove slashes from "///path"

Problem:    Simplify() does not remove slashes from "///path".
Solution:   Reduce > 2 slashes to one. (closes vim/vim#6263)

fdcbe3c3fe

Omit Test_readdirex() change: changed again in patch 9.0.0323.

Co-authored-by: Bram Moolenaar <Bram@vim.org>

src/nvim/path.c

@@ -1538,6 +1538,13 @@ void simplify_filename(char *filename)
     } while (vim_ispathsep(*p));
   }
   char *start = p;        // remember start after "c:/" or "/" or "///"
+#ifdef UNIX
+  // Posix says that "//path" is unchanged but "///path" is "/path".
+  if (start > filename + 2) {
+    STRMOVE(filename + 1, p);
+    start = p = filename + 1;
+  }
+#endif
 
   do {
     // At this point "p" is pointing to the char following a single "/"