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.

test/functional/ex_cmds/oldfiles_spec.lua

@@ -47,7 +47,7 @@ describe(':oldfiles', function()
   end)
 end)
 
-describe(':oldfiles!', function()
+describe(':browse oldfiles', function()
   local filename
   local filename2
   local oldfiles
@@ -74,7 +74,7 @@ describe(':oldfiles!', function()
     ok(filename == oldfiles[1] or filename == oldfiles[2])
     ok(filename2 == oldfiles[1] or filename2 == oldfiles[2])
 
-    execute('oldfiles!')
+    execute('browse oldfiles')
   end)
 
   after_each(function()