Add trailer support for commit creation

[Krishnachaitanyakc]

Summary

• Adds a trailers parameter to Commit.create_from_tree() and IndexFile.commit() enabling trailer creation (e.g. Signed-off-by, Issue) when committing programmatically
• Accepts either a dict or a list of (key, value) tuples (for duplicate keys like multiple Signed-off-by)
• Uses git interpret-trailers for formatting, consistent with existing trailer parsing in Commit.trailers_list

Closes #1998

Motivation

GitPython already supports parsing trailers from existing commits, but did not support adding them when creating new commits. The git commit CLI supports this via --trailer, e.g.:

git commit --message "Did a thing" --trailer "Issue: abc"

This change brings equivalent functionality to GitPython's Python API.

Usage

# Using a dict
repo.index.commit("Fix bug", trailers={"Issue": "123"})

# Using a list of tuples (for duplicate keys)
repo.index.commit("Fix bug", trailers=[
    ("Signed-off-by", "Alice <[email protected]>"),
    ("Signed-off-by", "Bob <[email protected]>"),
])

# Via Commit.create_from_tree directly
Commit.create_from_tree(repo, tree, "Fix bug", trailers={"Reviewed-by": "Eve"})

Test plan

• test_create_from_tree_with_trailers_dict - verifies dict-based trailer creation and round-trip parsing
• test_create_from_tree_with_trailers_list - verifies list-of-tuples trailer creation with duplicate keys
• test_index_commit_with_trailers - verifies IndexFile.commit() passes trailers through correctly
• All 3 new tests pass locally

[Byron]

@codex review

[Copilot]

Pull request overview

Adds support for creating commit trailers when programmatically generating commits, aligning commit creation with GitPython’s existing trailer parsing capabilities.

Changes:

• Add a trailers parameter to Commit.create_from_tree() to append trailers via git interpret-trailers.
• Thread trailers through IndexFile.commit() so index commits can include trailers.
• Add tests covering dict-based trailers, list-of-tuples trailers (duplicate keys), and the index commit plumbing.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
git/objects/commit.py	Adds trailers support in Commit.create_from_tree() by invoking git interpret-trailers.
git/index/base.py	Exposes trailers in IndexFile.commit() and forwards it to Commit.create_from_tree().
test/test_commit.py	Adds round-trip tests validating trailer creation and parsing for both APIs.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Byron]

Thanks a lot, this looks good to me.

The Copilot comments should be addressed and then it can be merged.

[Krishnachaitanyakc]

@Byron addressed those comments, can you please review?

[Byron]

It doesn't look like they are resolved, nor was anything pushed. Please also resolve them. Thank you.

[Krishnachaitanyakc]

@Byron sorry, missed pushing them. Can you review now?

[Byron]

Thanks a lot, looks good to me!

It's a shame that it's shelling out to build a string, but better to have a slow but correct version, than nothing at all in that regard.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

interpret-trailers I/O is hard-coded to UTF-8 (.encode() default + .decode('utf8')), but this method already resolves conf_encoding for the commit’s declared encoding. If i18n.commitencoding is not UTF-8, this can raise decode errors or corrupt non-ASCII characters; use the resolved encoding for both encoding stdin and decoding stdout (and consider an explicit error strategy).

Suggested change

	stdout_bytes, _ = proc.communicate(str(message).encode())
	finalize_process(proc)
	message = stdout_bytes.decode("utf8")
	stdout_bytes, _ = proc.communicate(str(message).encode(conf_encoding, errors="strict"))
	finalize_process(proc)
	message = stdout_bytes.decode(conf_encoding, errors="strict")

[Copilot]

This uses as_process=True, which makes Git.execute(..., with_exceptions=True) ineffective (see Git.execute docs), but the code never checks proc.returncode / stderr after communicate(). If interpret-trailers fails (e.g., unsupported --trailer on an older Git), message may be replaced with empty/partial stdout and the commit is still created; please detect non-zero exit status and raise (or preserve the original message on failure).

Suggested change

	)
	stdout_bytes, _ = proc.communicate(str(message).encode())
	finalize_process(proc)
	stderr=PIPE,
	)
	stdout_bytes, stderr_bytes = proc.communicate(str(message).encode())
	finalize_process(proc)
	if proc.returncode:
	stderr = stderr_bytes.decode("utf8", errors="replace") if stderr_bytes else ""
	raise RuntimeError(
	"git interpret-trailers failed with exit code "
	f"{proc.returncode}: {stderr or 'no error output'}"
	)

[Copilot]

The command uses repo.git.GIT_PYTHON_GIT_EXECUTABLE here, but Commit.trailers_list/trailers_dict currently invoke interpret-trailers via a hard-coded ['git', ...]. If a caller overrides GIT_PYTHON_GIT_EXECUTABLE, trailer creation and trailer parsing can end up using different Git binaries; consider aligning both paths on the same executable selection approach for consistent round-trips.

Suggested change

	cmd = [repo.git.GIT_PYTHON_GIT_EXECUTABLE, "interpret-trailers"] + trailer_args
	proc: Git.AutoInterrupt = repo.git.execute( # type: ignore[call-overload]
	cmd,
	proc: Git.AutoInterrupt = repo.git._call_process( # type: ignore[attr-defined]
	"interpret-trailers",
	*trailer_args,