A git commit object.

Commits are represented as a message plus an ordered list of headers. Preserving both the order and the verbatim content of headers is required for SHA stability: two commits with the same logical content but different header orderings are distinct git objects with distinct SHAs. This matters especially for signed commits — tampering with the header order would invalidate the gpgsig block.

Convenience accessors (tree/1, parents/1, author/1, committer/1, gpgsig/1) extract well-known headers. Arbitrary headers (e.g. encoding, mergetag, HG:rename) are preserved in the :headers list but have no dedicated accessor.