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 <alice@example.com>"),
    ("Signed-off-by", "Bob <bob@example.com>"),
])

# 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