Improve the readability of log --graph output

This series of patches are designed to improve the output of the log --graph command; their effect can be summed up in the following diagram:

    Before                    After
    ------                    -----

    *
    |\
    | *                       *
    | |\                      |\
    | | *                     | *
    | | |                     | |\
    | |  \                    | | *
    | *-. \                   | * |
    | |\ \ \                  |/|\|
    |/ / / /                  | | *
    | | | /                   | * |
    | | |/                    | |/
    | | *                     * /
    | * |                     |/
    | |/                      *
    * |
    |/
    *

These changes aim to make the edges in graph diagrams easier to read, by straightening lines and making certain kinds of topologies display more compactly. Three distinct changes are included.

First, if the first parent of a merge fuses with an edge to the left of the commit, then we display that by making the edges fuse immediately rather than by drawing a line straight down and then having it track to the left. That is, where we currently display these graphs:

    | *             | | | *
    | |\            | | | |\
    |/ /            | |_|/ /
    | |             |/| | |

We will now display these merges as follows:

    | *             | | | *
    |/|             | |_|/|
    | |             |/| | |

This transformation is applied to merges with any number of parents, for example we currently display 3-parent merges like this:

    | *-.           | | | *-.
    | |\ \          | | | |\ \
    |/ / /          | |_|/ / /
    | | |           |/| | | |

And we will now display them like this:

    | *             | | | *
    |/|\            | |_|/|\
    | | |           |/| | | |

If the edge the first parent fuses with is separated from the commit by multiple columns, a horizontal edge is drawn just as we currently do in the 'collapsing' state. This change also affects the display of commit and post-merge lines in subtle ways that are more thoroughly described in the relevant commits.

The second change is that if the final parent of a merge fuses with the edge to the right of the commit, then we can remove the zig-zag effect that currently results. We currently display these merges like this:

    * |
    |\ \
    | |/
    | *

After these changes, this merge will now be displayed like so:

    * |
    |\|
    | *

If the final parent fuses with an edge that's further to the right, its display is unchanged and it will still display like this:

    * | | |
    |\ \ \ \
    | | |_|/
    | |/| |
    | * | |

The final structural change smooths out lines that are collapsing through commit lines. For example, consider the following history:

    *-. \
    |\ \ \
    | | * |
    | * | |
    | |/ /
    * | |
    |/ /
    * |
    |/
    *

This is now rendered so that commit lines display an edge using / instead of |, if that edge is tracking to the left both above and below the commit line. That results in this improved display:

    *-. \
    |\ \ \
    | | * |
    | * | |
    | |/ /
    * / /
    |/ /
    * /
    |/
    *

Taken together, these changes produce the change shown in the first diagram above, with the current rendering on the left and the new rendering on the right.

A final addition to that set of changes fixes the coloring of dashes that are drawn next to octopus merges, in a manner compatible with all these changes. The early commits in this set are refactorings that make the functional changes easier to introduce.

James Coglan (13):
graph: automatically track display width of graph lines
graph: handle line padding in graph_next_line()
graph: reuse find_new_column_by_commit()
graph: reduce duplication in graph_insert_into_new_columns()
graph: remove mapping_idx and graph_update_width()
graph: extract logic for moving to GRAPH_PRE_COMMIT state
graph: example of graph output that can be simplified
graph: tidy up display of left-skewed merges
graph: commit and post-merge lines for left-skewed merges
graph: rename new_mapping to old_mapping
graph: smooth appearance of collapsing edges on commit lines
graph: flatten edges that fuse with their right neighbor
graph: fix coloring of octopus dashes

 graph.c                                    | 646 ++++++++++++---------
 t/t3430-rebase-merges.sh                   |   2 +-
 t/t4202-log.sh                             |   2 +-
 t/t4214-log-graph-octopus.sh               |  62 +-
 t/t4215-log-skewed-merges.sh               | 257 ++++++++
 t/t6016-rev-list-graph-simplify-history.sh |  30 +-
 6 files changed, 673 insertions(+), 326 deletions(-)
 create mode 100755 t/t4215-log-skewed-merges.sh

base-commit: 108b97dc372828f0e72e56bbb40cae8e1e83ece6
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-383%2Fjcoglan%2Fjc%2Fsimplify-graph-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-383/jcoglan/jc/simplify-graph-v3
Pull-Request: https://github.com/gitgitgadget/git/pull/383

Range-diff vs v2:

  1:  c30f5bb43b =  1:  722ab8973a graph: automatically track display width of graph lines
  2:  d962eb2e02 !  2:  056c95d4ed graph: handle line padding in `graph_next_line()`
     @@ -18,6 +18,8 @@
          `graph_next_line()` and we must not pad the `graph_line` if no commit is
          set.
      
     +    Signed-off-by: James Coglan <[email protected]>
     +
       diff --git a/graph.c b/graph.c
       --- a/graph.c
       +++ b/graph.c
  3:  ecfcfbfd5c =  3:  376236ab0b graph: reuse `find_new_column_by_commit()`
  4:  50ce875ed9 =  4:  6a4717bf05 graph: reduce duplication in `graph_insert_into_new_columns()`
  5:  d2e8958eed =  5:  678b53b671 graph: remove `mapping_idx` and `graph_update_width()`
  6:  163600585c =  6:  8ed32b1c74 graph: extract logic for moving to GRAPH_PRE_COMMIT state
  7:  51495be940 !  7:  631ee3cecb graph: example of graph output that can be simplified
     @@ -86,6 +86,8 @@
                  |/
                  * A
      
     +    Signed-off-by: James Coglan <[email protected]>
     +
       diff --git a/t/t4215-log-skewed-merges.sh b/t/t4215-log-skewed-merges.sh
       new file mode 100755
       --- /dev/null
  8:  2ab0f9775b =  8:  c34a5eb160 graph: tidy up display of left-skewed merges
  9:  0e37c88c60 =  9:  2f75c697be graph: commit and post-merge lines for left-skewed merges
 10:  9bbf738e6d = 10:  f5c2791436 graph: rename `new_mapping` to `old_mapping`
 11:  67051ec31a = 11:  9b24893de6 graph: smooth appearance of collapsing edges on commit lines
 12:  503c846d2b = 12:  04ba5169bc graph: flatten edges that fuse with their right neighbor
 13:  07ddd509c5 = 13:  cd761b3a32 graph: fix coloring of octopus dashes

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