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/starting.txt

@@ -1200,11 +1200,10 @@ running) you have additional options:
 :wv[iminfo][!] [file]	Deprecated alias to |:wshada| command.
 
 						*:o* *:ol* *:oldfiles*
-:o[ldfiles][!]		List the files that have marks stored in the ShaDa
+:o[ldfiles]		List the files that have marks stored in the ShaDa
 			file.  This list is read on startup and only changes
 			afterwards with ":rshada!".  Also see |v:oldfiles|.
 			The number can be used with |c_#<|.
-			Use ! to get a file selection prompt.
 
 :bro[wse] o[ldfiles][!]
 			List file names as with |:oldfiles|, and then prompt

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|

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!"
+    // File selection prompt on ":browse oldfiles"
-    if (eap->forceit) {
+    if (cmdmod.browse) {
       quit_more = false;
       nr = prompt_for_number(false);
       msg_starthere();

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()