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.

src/nvim/eval.c

@@ -21722,8 +21722,8 @@ void ex_oldfiles(exarg_T *eap)
     /* Assume "got_int" was set to truncate the listing. */
     got_int = FALSE;
 
-    // File selection prompt on ":oldfiles!"
-    if (eap->forceit) {
+    // File selection prompt on ":browse oldfiles"
+    if (cmdmod.browse) {
       quit_more = false;
       nr = prompt_for_number(false);
       msg_starthere();