Add a test for loading an example undofile

This is so, that we don't unintentionally change the undofile format and
make sure we can reload an older undo file.

Filelist

@@ -224,6 +224,7 @@ SRC_ALL =	\
 		src/testdir/samples/*.vim \
 		src/testdir/samples/test000 \
 		src/testdir/samples/test.zip \
+		src/testdir/samples/test_undo.txt.undo \
 		src/testdir/samples/testa.zip \
 		src/testdir/color_ramp.vim \
 		src/testdir/silent.wav \

src/testdir/samples/test_undo.txt

@@ -0,0 +1,3 @@
+one
+two
+three

src/testdir/test_undo.vim

@@ -884,4 +884,33 @@ func Test_undo_range_normal()
   bwipe!
 endfunc
 
+func Test_load_existing_undofile()
+  CheckFeature persistent_undo
+  sp samples/test_undo.txt
+  let mess=execute(':verbose rundo samples/test_undo.txt.undo')
+  call assert_match('Finished reading undo file', mess)
+
+  call assert_equal(['one', 'two', 'three'], getline(1, '$'))
+  norm! u
+  call assert_equal(['one', 'two'], getline(1, '$'))
+  norm! u
+  call assert_equal(['one'], getline(1, '$'))
+  norm! u
+  call assert_equal([''], getline(1, '$'))
+  let mess = execute(':norm! u')
+  call assert_equal([''], getline(1, '$'))
+  call assert_match('Already at oldest change', mess)
+  exe ":norm! \<c-r>"
+  call assert_equal(['one'], getline(1, '$'))
+  exe ":norm! \<c-r>"
+  call assert_equal(['one', 'two'], getline(1, '$'))
+  exe ":norm! \<c-r>"
+  call assert_equal(['one', 'two', 'three'], getline(1, '$'))
+  let mess = execute(":norm! \<c-r>")
+  call assert_equal(['one', 'two', 'three'], getline(1, '$'))
+  call assert_match('Already at newest change', mess)
+  bw!
+endfunc
+
+
 " vim: shiftwidth=2 sts=2 expandtab