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

@@ -1189,22 +1189,30 @@ func Test_diff_followwrap()
 endfunc
 
 func Test_diff_maintains_change_mark()
-  enew!
-  call setline(1, ['a', 'b', 'c', 'd'])
-  diffthis
-  new
-  call setline(1, ['a', 'b', 'c', 'e'])
-  " Set '[ and '] marks
-  2,3yank
-  call assert_equal([2, 3], [line("'["), line("']")])
-  " Verify they aren't affected by the implicit diff
-  diffthis
-  call assert_equal([2, 3], [line("'["), line("']")])
-  " Verify they aren't affected by an explicit diff
-  diffupdate
-  call assert_equal([2, 3], [line("'["), line("']")])
-  bwipe!
-  bwipe!
+  func DiffMaintainsChangeMark()
+    enew!
+    call setline(1, ['a', 'b', 'c', 'd'])
+    diffthis
+    new
+    call setline(1, ['a', 'b', 'c', 'e'])
+    " Set '[ and '] marks
+    2,3yank
+    call assert_equal([2, 3], [line("'["), line("']")])
+    " Verify they aren't affected by the implicit diff
+    diffthis
+    call assert_equal([2, 3], [line("'["), line("']")])
+    " Verify they aren't affected by an explicit diff
+    diffupdate
+    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,
 /**/