undo: :earlier, g-: Set b_u_seq_cur correctly. (#6016)

Previously alternate branches were not accounted for properly, with this
change g- after an undo to a branch point works.

The current sequence number b_u_seq_cur is used in undo_time(), in
u_doit() this was calculated by subtracting one from the curhead
sequence number.

The curhead header entry represents the change that was just undone, so
the sequence number we want is that of the change we have moved to. This
is the sequence number of the undo head that is the uh_next element of
this curhead. That sequence number is not always one less than the
curhead sequence number -- there may have been an alternate branch at
this point.

Instead of subtracting one, we now directly find the sequence number of
curhead->uh_next.

src/nvim/undo.c

@@ -1710,7 +1710,8 @@ bool u_undo_and_forget(int count)
   if (curbuf->b_u_curhead) {
     to_forget->uh_alt_next.ptr = NULL;
     curbuf->b_u_curhead->uh_alt_prev.ptr = to_forget->uh_alt_prev.ptr;
-    curbuf->b_u_seq_cur = curbuf->b_u_curhead->uh_seq-1;
+    curbuf->b_u_seq_cur = curbuf->b_u_curhead->uh_next.ptr ?
+        curbuf->b_u_curhead->uh_next.ptr->uh_seq : 0;
   } else if (curbuf->b_u_newhead) {
     curbuf->b_u_seq_cur = curbuf->b_u_newhead->uh_seq;
   }
@@ -2321,7 +2322,8 @@ static void u_undoredo(int undo)
   if (undo)
     /* We are below the previous undo.  However, to make ":earlier 1s"
      * work we compute this as being just above the just undone change. */
-    --curbuf->b_u_seq_cur;
+    curbuf->b_u_seq_cur = curhead->uh_next.ptr ?
+        curhead->uh_next.ptr->uh_seq : 0;
 
   /* Remember where we are for ":earlier 1f" and ":later 1f". */
   if (curhead->uh_save_nr != 0) {

test/functional/helpers.lua

@@ -195,7 +195,7 @@ local function dedent(str)
       indent = line_indent
     end
   end
-  if #indent == 0 then
+  if indent == nil or #indent == 0 then
     -- no minimum common indent
     return str
   end

test/functional/normal/undo_spec.lua

@@ -0,0 +1,61 @@
+local helpers = require('test.functional.helpers')(after_each)
+
+local clear = helpers.clear
+local execute = helpers.execute
+local expect = helpers.expect
+local feed = helpers.feed
+local insert = helpers.insert
+
+describe('u CTRL-R g- g+', function()
+  before_each(clear)
+
+  local function create_history(num_steps)
+    if num_steps == 0 then return end
+    insert('1')
+    if num_steps == 1 then return end
+    feed('o2<esc>')
+    feed('o3<esc>')
+    feed('u')
+    if num_steps == 2 then return end
+    feed('o4<esc>')
+    if num_steps == 3 then return end
+    feed('u')
+  end
+
+  local function undo_and_redo(hist_pos, undo, redo, expect_str)
+    execute('enew!')
+    create_history(hist_pos)
+    local cur_contents = helpers.curbuf_contents()
+    feed(undo)
+    expect(expect_str)
+    feed(redo)
+    expect(cur_contents)
+  end
+
+  -- TODO Look for message saying 'Already at oldest change'
+  it('does nothing when no changes have happened', function()
+    undo_and_redo(0, 'u', '<C-r>', '')
+    undo_and_redo(0, 'g-', 'g+', '')
+  end)
+  it('undoes a change when at a leaf', function()
+    undo_and_redo(1, 'u', '<C-r>', '')
+    undo_and_redo(1, 'g-', 'g+', '')
+  end)
+  it('undoes a change when in a non-leaf', function()
+    undo_and_redo(2, 'u', '<C-r>', '1')
+    undo_and_redo(2, 'g-', 'g+', '1')
+  end)
+  it('undoes properly around a branch point', function()
+    undo_and_redo(3, 'u', '<C-r>', [[
+      1
+      2]])
+    undo_and_redo(3, 'g-', 'g+', [[
+      1
+      2
+      3]])
+  end)
+  it('can find the previous sequence after undoing to a branch', function()
+    undo_and_redo(4, 'u', '<C-r>', '1')
+    undo_and_redo(4, 'g-', 'g+', '1')
+  end)
+end)

test/functional/ui/inccommand_spec.lua

@@ -352,6 +352,14 @@ describe(":substitute, 'inccommand' preserves undo", function()
 
     feed("2u")
     feed(substring .. "<esc>")
+    expect([[
+      1]])
+    feed("g-")
+    expect([[
+      ]])
+    feed("g+")
+    expect([[
+      1]])
     feed("<c-r>")
     expect([[
       1
@@ -374,9 +382,7 @@ describe(":substitute, 'inccommand' preserves undo", function()
       2]])
   end
 
-  -- TODO(vim): This does not work, even in Vim.
-  -- Waiting for fix (perhaps from upstream).
-  pending("at a non-leaf of the undo tree", function()
+  it("at a non-leaf of the undo tree", function()
    for _, case in pairs(cases) do
      for _, str in pairs(substrings) do
        for _, redoable in pairs({true}) do