Re: [PATCH] revision: --include-diversions adds helpful merges

[Jeff King <peff@peff.net>]

On Wed, Apr 08, 2020 at 01:22:03AM +0000, Derrick Stolee via GitGitGadget wrote:

> The default file history simplification of "git log -- <path>" or
> "git rev-list -- <path>" focuses on providing the smallest set of
> commits that first contributed a change. The revision walk greatly
> restricts the set of walked commits by visiting only the first
> TREESAME parent of a merge commit, when one exists. This means
> that portions of the commit-graph are not walked, which can be a
> performance benefit, but can also "hide" commits that added changes
> but were ignored by a merge resolution.
> [...]

Thanks for a really great description of the problem.

Playing around with the patch, I found one curiosity. Try this:

  git log --graph --oneline origin -- GIT-VERSION-GEN >old
  git log --graph --oneline --include-diversions \
                            origin -- GIT-VERSION-GEN >new
  diff -u old new

The first hunk has:

  @@ -70,6 +70,7 @@
   * 20769079d2 Git 2.12-rc2
   * 5588dbffbd Git 2.12-rc1
   * 6e3a7b3398 Git 2.12-rc0
  +* 0a45050a14 Merge branch 'rj/git-version-gen-do-not-force-abbrev'
   * a7659747c2 GIT-VERSION-GEN: do not force abbreviation length used by 'describe'
   * 8d7a455ed5 Start post 2.11 cycle
   * 454cb6bd52 Git 2.11

which makes sense. That merge brought in a7659747c2, and the other side
hadn't touched it. But I can't tell from the output how the two are
related. Nor can I just add "-p" to the invocation; after we've
simplified, it has only a single parent, but it's TREESAME to that
parent. So it has no diff.

I actually think the most descriptive output here would be something
like:

  * 6e3a7b3398 Git 2.12-rc0
  * 0a45050a14 Merge branch 'rj/git-version-gen-do-not-force-abbrev'
  |\
  | * a7659747c2 GIT-VERSION-GEN: do not force abbreviation length used by 'describe'
  |/
  * Start post 2.11 cycle

I.e., leaving both parents intact for a "diversion" merge. But maybe
that would have secondary effects in other places.

-Peff