Restore ":browse oldfiles" behavior from Vim

In 3b12bb225a, ":oldfiles" was taught to
behave like Vim's ":browse oldfiles" if ":oldfiles!" was used.  However,
this conflates the use of ! for abandoning a modified buffer with
choosing one file out of a list of oldfiles.

Now that ":browse" is supported again, ":browse oldfiles" will allow the
user to select an old file, while still complaining if that would cause
a modified buffer to be abandoned.  ":browse oldfiles!" will just
abandon the buffer, as expected.

runtime/doc/vim_diff.txt

@@ -188,7 +188,6 @@ Additional differences:
   compatibility reasons.
 - |:wviminfo| was renamed to |:wshada|, |:rviminfo| to |:rshada|.  Old
   commands are still kept.
-- |:oldfiles| supports !.
 - When writing (|:wshada| without bang or at exit) it merges much more data,
   and does this according to the timestamp.  Vim merges only marks.
   |shada-merging|