use top bottom and middle commit symbols

I like to use ┌ ├ └ for the commits, because it gives
me lines to follow visually. With flog v1 this works:

Plug 'rbong/vim-flog', { 'branch': 'v1' }
Plug 'TamaMcGlinn/flog-forest', { 'branch': 'v1' }

But with v2 the ability to customize the forest used in the background
was lost, so I have re-added this by checking if the commit has child
commits already drawn, and whether it has children that will later be
drawn, and still falling back to the • if neither (disconnected
commits).

The change to the MergeStart line just makes it a little more strict,
as it must be followed by a non-space character such as ──╮.
Otherwise, it thinks each middle-commit line is actually a
mergestart line and those commit lines become all-white.

lua/flog/graph.lua

@@ -5,7 +5,10 @@
 local graph_error = 'flog: internal error drawing graph'
 
 -- Init graph strings
-local current_commit_str = '• '
+local top_commit_str = '┌ '
+local middle_commit_str = '├ '
+local bottom_commit_str = '└ '
+local disconnected_commit_str = '• '
 local commit_branch_str = '│ '
 local commit_empty_str = ' '
 local complex_merge_str_1 = '┬┊'
@@ -28,6 +31,21 @@ local missing_parent_str = '┊ '
 local missing_parent_branch_str = '│ '
 local missing_parent_empty_str = ' '
 
+local function get_commit_string(is_top, is_bottom)
+ if not is_top and not is_bottom then
+ return middle_commit_str
+ end
+ if is_top and is_bottom then
+ return disconnected_commit_str
+ end
+ if is_top then
+ return top_commit_str
+ end
+ if is_bottom then
+ return bottom_commit_str
+ end
+end
+
 local function flog_get_graph(
  enable_vim,
  enable_nvim,
@@ -122,6 +140,8 @@ local function flog_get_graph(
  local branch_hashes = {}
  local branch_indexes = {}
  local nbranches = 0
+ -- set of commits that have children already printed
+ local commit_hashes_with_children = {}
 
  -- Draw graph
 
@@ -201,6 +221,11 @@ local function flog_get_graph(
  parent_index = parent_index + 1
  end
 
+ -- Mark parent commits as mentioned
+ for _, parent_hash in ipairs(parents) do
+ commit_hashes_with_children[parent_hash] = true
+ end
+
  -- Traverse old and new branches
 
  if enable_graph then
@@ -227,7 +252,13 @@ local function flog_get_graph(
  is_commit = true
  end
 
+ local current_commit_str
  if is_commit then
+ -- identify symbol to draw for commit
+ local is_top = commit_hashes_with_children[commit_hash] == nil
+ local is_bottom = nparents == 0
+ current_commit_str = get_commit_string(is_top, is_bottom)
+
  -- Count commit merge
  nmerges_right = nmerges_right - 1
  nmerges_left = nmerges_left + 1
@@ -294,7 +325,7 @@ local function flog_get_graph(
  branch_indexes[parent_hash] = branch_index
  branch_hashes[branch_index] = parent_hash
 
- -- Update branch has
+ -- Update branch hash
  branch_hash = parent_hash
 
  -- Update the number of branches

syntax/floggraph.vim

@@ -134,15 +134,15 @@ for branch_idx in range(1, 9)
  exec 'syntax match ' . branch . ' contained nextgroup=' . next_branch . ',' . next_branch . 'Commit,' . next_branch . 'MergeStart,' . next_branch . 'ComplexMergeStart,' . next_branch . 'MissingParentsStart,flogCollapsedCommit,@flogDiff /\v%( |%u2502 |%u2502$)/'
 
  " Commit indicators
- exec 'syntax match ' . branch . 'Commit contained nextgroup=' . next_branch . 'AfterCommit,@flogCommitInfo /\%u2022 /'
+ exec 'syntax match ' . branch . 'Commit contained nextgroup=' . next_branch . 'AfterCommit,@flogCommitInfo /\v%(%u2022|%u250c|%u251c|%u2514) /'
  exec 'highlight link ' . branch . 'Commit flogCommit'
 
  " Branches to the right of the commit indicator
  exec 'syntax match ' . branch . 'AfterCommit contained nextgroup=' . next_branch . 'AfterCommit,@flogCommitInfo /\v%( |%u2502 |%u2502$)/'
  exec 'highlight link ' . branch . 'AfterCommit ' . branch
 
  " Start of a merge - saves the branch that the merge starts on (see below)
- exec 'syntax match ' . branch . 'MergeStart contained nextgroup=' . next_merge_branch . ',' . next_merge_branch . 'End /\v%(%u251c|%u256d|%u2570)/'
+ exec 'syntax match ' . branch . 'MergeStart contained nextgroup=' . next_merge_branch . ',' . next_merge_branch . 'End /\v%(%u251c|%u256d|%u2570)[^ ]/'
  exec 'highlight link ' . branch . 'MergeStart ' . branch
 
  " Horizontal merge character