Merge branch 'jc/log-graph-simplify'

The implementation of "git log --graph" got refactored and then its
output got simplified.

* jc/log-graph-simplify:
  t4215: use helper function to check output
  graph: fix coloring of octopus dashes
  graph: flatten edges that fuse with their right neighbor
  graph: smooth appearance of collapsing edges on commit lines
  graph: rename `new_mapping` to `old_mapping`
  graph: commit and post-merge lines for left-skewed merges
  graph: tidy up display of left-skewed merges
  graph: example of graph output that can be simplified
  graph: extract logic for moving to GRAPH_PRE_COMMIT state
  graph: remove `mapping_idx` and `graph_update_width()`
  graph: reduce duplication in `graph_insert_into_new_columns()`
  graph: reuse `find_new_column_by_commit()`
  graph: handle line padding in `graph_next_line()`
  graph: automatically track display width of graph lines

Author: gitster