patch 8.0.0149: :earlier does not work after reading the undo file

Problem:    ":earlier" and ":later" do not work after startup or reading the
            undo file.
Solution:   Use absolute time stamps instead of relative to the Vim start
            time. (Christian Brabandt, Pavel Juhas, closes #1300, closes
            #1254)

src/testdir/test_undo.vim

@@ -235,3 +235,31 @@ func Test_insert_expr()
 
   close!
 endfunc
+
+func Test_undofile_earlier()
+  " Issue #1254
+  " create undofile with timestamps older than Vim startup time.
+  let t0 = localtime() - 43200
+  call test_settime(t0)
+  new Xfile
+  call feedkeys("ione\<Esc>", 'xt')
+  set ul=100
+  call test_settime(t0 + 1)
+  call feedkeys("otwo\<Esc>", 'xt')
+  set ul=100
+  call test_settime(t0 + 2)
+  call feedkeys("othree\<Esc>", 'xt')
+  set ul=100
+  w
+  wundo Xundofile
+  bwipe!
+  " restore normal timestamps.
+  call test_settime(0)
+  new Xfile
+  rundo Xundofile
+  earlier 1d
+  call assert_equal('', getline(1))
+  bwipe!
+  call delete('Xfile')
+  call delete('Xundofile')
+endfunc

src/undo.c

@@ -2298,10 +2298,8 @@ undo_time(
     }
     else
     {
-	/* When doing computations with time_t subtract starttime, because
-	 * time_t converted to a long may result in a wrong number. */
 	if (dosec)
-	    target = (long)(curbuf->b_u_time_cur - starttime) + step;
+	    target = (long)(curbuf->b_u_time_cur) + step;
 	else if (dofile)
 	{
 	    if (step < 0)
@@ -2350,7 +2348,7 @@ undo_time(
 	else
 	{
 	    if (dosec)
-		closest = (long)(vim_time() - starttime + 1);
+		closest = (long)(vim_time() + 1);
 	    else if (dofile)
 		closest = curbuf->b_u_save_nr_last + 2;
 	    else
@@ -2388,7 +2386,7 @@ undo_time(
 	{
 	    uhp->uh_walk = mark;
 	    if (dosec)
-		val = (long)(uhp->uh_time - starttime);
+		val = (long)(uhp->uh_time);
 	    else if (dofile)
 		val = uhp->uh_save_nr;
 	    else

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    149,
 /**/
     148,
 /**/