vim-patch:8.2.0893: assert_equalfile() does not take a third argument

Problem:    Assert_equalfile() does not take a third argument.
Solution:   Implement the third argument. (Gary Johnson)
fb517bac23

src/nvim/eval.c

@@ -5739,6 +5739,12 @@ int assert_equalfile(typval_T *argvars)
   }
   if (IObuff[0] != NUL) {
     prepare_assert_error(&ga);
+    if (argvars[2].v_type != VAR_UNKNOWN) {
+      char *const tofree = encode_tv2echo(&argvars[2], NULL);
+      ga_concat(&ga, (char_u *)tofree);
+      xfree(tofree);
+      ga_concat(&ga, (char_u *)": ");
+    }
     ga_concat(&ga, IObuff);
     assert_error(&ga);
     ga_clear(&ga);

src/nvim/eval.lua

@@ -28,7 +28,7 @@ return {
     asin={args=1, func="float_op_wrapper", data="&asin"},  -- WJMc
     assert_beeps={args={1, 2}},
     assert_equal={args={2, 3}},
-    assert_equalfile={args=2},
+    assert_equalfile={args={2, 3}},
     assert_exception={args={1, 2}},
     assert_fails={args={1, 3}},
     assert_false={args={1, 2}},

src/nvim/eval/funcs.c

@@ -415,7 +415,7 @@ static void f_assert_equal(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   rettv->vval.v_number = assert_equal_common(argvars, ASSERT_EQUAL);
 }
 
-// "assert_equalfile(fname-one, fname-two)" function
+// "assert_equalfile(fname-one, fname-two[, msg])" function
 static void f_assert_equalfile(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 {
   rettv->vval.v_number = assert_equalfile(argvars);

src/nvim/testdir/test_assert.vim

@@ -31,6 +31,10 @@ func Test_assert_equalfile()
   call assert_match("difference at byte 4", v:errors[0])
   call remove(v:errors, 0)
 
+  call assert_equal(1, assert_equalfile('Xone', 'Xtwo', 'a message'))
+  call assert_match("a message: difference at byte 4", v:errors[0])
+  call remove(v:errors, 0)
+
   call delete('Xone')
   call delete('Xtwo')
 endfunc