[PATCH 1/4] sequencer: Do not require `allow_empty` for redundant commit options

From: Brian Lyles <brianmlyles@gmail.com>

Previously, a consumer of the sequencer that wishes to take advantage of either the `keep_redundant_commits` or `drop_redundant_commits` feature must also specify `allow_empty`.

The only consumer of `drop_redundant_commits` is `git-rebase`, which already allows empty commits by default and simply always enables `allow_empty`. `keep_redundant_commits` was also consumed by `git-cherry-pick`, which had to specify `allow-empty` when `keep_redundant_commits` was specified in order for the sequencer's `allow_empty()` to actually respect `keep_redundant_commits`.

The latter is an interesting case: As noted in the docs, this means that `--keep-redundant-commits` implies `--allow-empty`, despite the two having distinct, non-overlapping meanings:

- `allow_empty` refers specifically to commits which start empty, as
  indicated by the documentation for `--allow-empty` within
  `git-cherry-pick`:

  "Note also, that use of this option only keeps commits that were
  initially empty (i.e. the commit recorded the same tree as its
  parent). Commits which are made empty due to a previous commit are
  dropped. To force the inclusion of those commits use
  --keep-redundant-commits."

- `keep_redundant_commits` refers specifically to commits that do not
  start empty, but become empty due to the content already existing in
  the target history. This is indicated by the documentation for
  `--keep-redundant-commits` within `git-cherry-pick`:

  "If a commit being cherry picked duplicates a commit already in the
  current history, it will become empty. By default these redundant
  commits cause cherry-pick to stop so the user can examine the commit.
  This option overrides that behavior and creates an empty commit
  object. Implies --allow-empty."

This implication of `--allow-empty` therefore seems incorrect: One should be able to keep a commit that becomes empty without also being forced to pick commits that start as empty. However, today, the following series of commands would result in both the commit that became empty and the commit that started empty being picked despite only `--keep-redundant-commits` being specified:

    git init
    echo "a" >test
    git add test
    git commit -m "Initial commit"
    echo "b" >test
    git commit -am "a -> b"
    git commit --allow-empty -m "empty"
    git cherry-pick --keep-redundant-commits HEAD^ HEAD

The same cherry-pick with `--allow-empty` would fail on the redundant commit, and with neither option would fail on the empty commit.

In a future commit, an `--empty` option will be added to `git-cherry-pick`, meaning that `drop_redundant_commits` will be available in that command. For that to be possible with the current implementation of the sequencer's `allow_empty()`, `git-cherry-pick` would need to specify `allow_empty` with `drop_redundant_commits` as well, which is an even less intuitive implication of `--allow-empty`: in order to prevent redundant commits automatically, initially-empty commits would need to be kept automatically.

Instead, this commit rewrites the `allow_empty()` logic to remove the over-arching requirement that `allow_empty` be specified in order to reach any of the keep/drop behaviors. Only if the commit was originally empty will `allow_empty` have an effect.

For some amount of backwards compatibility with the existing code and tests, I have opted to preserve the behavior of returning 0 when:

- `allow_empty` is specified, and
- either `is_index_unchanged` or `is_original_commit_empty` indicates an
  error

This is primarily out of caution -- I am not positive what downstream impacts this might have.

Note that this commit is a breaking change: `--keep-redundant-commits` will no longer imply `--allow-empty`. It would be possible to maintain the current behavior of `--keep-redundant-commits` implying `--allow-empty` if it were needed to avoid a breaking change, but I believe that decoupling them entirely is the correct behavior.

Signed-off-by: Brian Lyles <brianmlyles@gmail.com>
---

Disclaimer: This is my first contribution to the git project, and thus
my first attempt at submitting a patch via `git-send-email`. It is also
the first time I've touched worked in C in over a decade, and I really
didn't work with it much before that either. I welcome any and all
feedback on what I may have gotten wrong regarding the patch submission
process, the code changes, or my commit messages.

This is the first in a series of commits that aims to introduce an `--empty` option to `git-cherry-pick` that provides the same flexibility as the `--empty` options for `git-rebase` and `git-am`, as well as improve the consistency in the values and documentation for this option across the three commands.

The main thing that may be controversial with this particular commit is that I am proposing a breaking change. As described in the above message, I do not think that it makes sense to tie `--allow-empty` and `--keep-redundant-commits` together since they appear to be intended to work with different types of empty commits. That being said, if it is deemed unacceptable to make this breaking change, we can consider an alternative approach where we maintain the behavior of `--keep-redundant-commits` implying `--allow-empty`, while preventing the need for the future `--empty=drop` to have that same implication.

 Documentation/git-cherry-pick.txt | 10 +++++++---
 builtin/revert.c                  |  4 ----
 sequencer.c                       | 18 ++++++++++--------
 t/t3505-cherry-pick-empty.sh      |  5 +++++
 4 files changed, 22 insertions(+), 15 deletions(-)