patch 8.2.3936: no proper test for maintaining change mark in diff mode

Problem:    No proper test for maintaining change mark in diff mode.
Solution:   Run the test with internal and external diff. (Sean Dewar,
            closes #9424)

src/testdir/test_diffmode.vim

@@ -1189,6 +1189,7 @@ func Test_diff_followwrap()
 endfunc
 
 func Test_diff_maintains_change_mark()
+  func DiffMaintainsChangeMark()
     enew!
     call setline(1, ['a', 'b', 'c', 'd'])
     diffthis
@@ -1205,6 +1206,13 @@ func Test_diff_maintains_change_mark()
     call assert_equal([2, 3], [line("'["), line("']")])
     bwipe!
     bwipe!
+  endfunc
+
+  set diffopt-=internal
+  call DiffMaintainsChangeMark()
+  set diffopt+=internal
+  call DiffMaintainsChangeMark()
+  set diffopt&
 endfunc
 
 " Test for 'patchexpr'

src/version.c

@@ -749,6 +749,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3936,
 /**/
     3935,
 /**/