[GSoC PATCH v3 0/1] graph: add indentation for commits preceded by a parentless commit

When having a history with multiple root commits or commits that act like roots (they have excluded parents), let's call them parentless, and drawing the history near them, the graphing engine renders the commits one below the other, seeming that they are related:

  * parentless A
  * child B
  * parentless B

This issue has been attempted multiple times:
  https://lore.kernel.org/git/xmqqwnwajbuj.fsf@gitster.c.googlers.com/

This happens because the engine prints left to right from the first free column and their column of these parentless commits for the next row becomes empty and the engine fills that gap with the next commit (child B) seeming that parentless A and child B are related when they are not.

The actual implementation is very minimal. This patch makes the parentless commits to be kept alive at least one more row to avoid that, indenting the next commit to the next column and then clean the mapping letting the indented commit to naturally collapse to the column where the parentless commit was:

  * parentless A
    * child B
   /
  * parentless B

This is done by adding a is_placeholder flag to the columns, the parentless commit is actually there but marked as a placeholder:

   * parentless A
  (A) * child B
    /
   * parentless B

(A) would be "parentless A" column with the placeholder flag active.

By teaching the rendering function to print a padding ' ' when meeting a placeholder column hides them, printing the second example.

There could also be the case where there are multiple parentless commits

without the patch:

  * parentless A
  * parentless B
  * parentless C
  * child D
  * parentless D

with the patch, the indentation cascades:

  * parentless A
    * parentless B
      * parentless C
        * child D
     _ /
    /
   /
  * parentless D

the _ / might look weird but that's how the collapsing rendering does it for big gaps, this case being from the 4th column to the 0th column.

A follow-up could change the collapsing rendering for placeholders? I haven't done it to keep it minimal, but a follow up could make it to be straight '/'. This would make it bigger but easier for the eye to follow. Is not worth it IMO, but opinions are welcome.

The patch also adds tests for different cases like a parentless commit preceding multiple parents merges and the examples above.

PSA: the tests are on t4215-log-skewed-merges.sh, which is not very related,
     but other graph related tests have +140 tests, and this one has less than
     20 and some of them are also not very related and differ in style.
     A cleanup patch before this renaming the file and style of the tests is fine?

Changes from v2:

- Removed trailing whitespace.
- Added more comments to make it more clear an reviewable.
- Changed is_root to is_parentless to follow the name at the cover letter and
  commit.
- simplified cover letter and commit ascii graphs.

Pablo Sabater (1):
  graph: add indentation for commits preceded by a parentless commit

 graph.c                      | 115 ++++++++++++++++++++++++++++++--
 t/t4215-log-skewed-merges.sh | 124 +++++++++++++++++++++++++++++++++++
 2 files changed, 233 insertions(+), 6 deletions(-)

base-commit: 94f057755b7941b321fd11fec1b2e3ca5313a4e0 -- 2.43.0