Signed-off-by: Paulo Gomes <[email protected]>

Signed-off-by: Paulo Gomes <[email protected]>

Replace the per-iteration flag soup in (*Commit).Decode with a
func-based stateFn loop that mirrors upstream Git's parsing posture.
The new state machine encodes "where we are in the buffer" as the
function being called rather than as a bag of mutable booleans.

Conformance changes vs upstream commit.c:
- tree must be the first header, missing tree errors with
  ErrMalformedCommit (matches the "bogus commit object" check at
  commit.c:508-510).
- Parents are accepted only contiguously after tree; out-of-position
  parent lines are silently dropped (matches parse_commit_buffer's
  parent loop exiting at the first non-parent line and
  read_commit_extra_header_lines filtering parents from extras).
- Author and committer are accepted only at their canonical positions
  (immediately after parents, immediately after author). Out-of-place
  occurrences are dropped, mirroring parse_commit_date silently
  returning 0 plus the standard_header_field filter.
- Encoding/mergetag/gpgsig/gpgsig-sha256 follow first-wins; duplicate
  sig/mergetag continuation lines are discarded by a dedicated
  scanSkipCont state.
- Continuation strip uses line[1:] (single space) instead of
  bytes.TrimLeft, mirroring upstream's `line + 1` at commit.c:1509.
- Header/body boundary is the literal "\n" only, matching
  *line == '\n' at commit.c:1502.

Tests:
- TestDecodeRequiresTreeFirst covers the four hard-error inputs
  (missing tree, parent before tree, extra before tree, empty buffer).
- TestDecodeFirstOccurrenceWins covers the lenient drop semantics for
  duplicate tree/author/committer/gpgsig/gpgsig-sha256, parents after
  author or interleaved with extras, missing committer, encoding
  between author and committer (drops the misplaced committer), and
  author at a non-canonical position (drops author and committer).
- TestMalformedHeader is back to expecting NoError (lenient parsing of
  garbage author/committer values).

The state machine lives in plumbing/object/commit_scanner.go.

Assisted-by: Claude Opus 4.7 <[email protected]>
Signed-off-by: Paulo Gomes <[email protected]>

The previous MergeTag field had two issues:
1. It could only represent a single mergetag, whilst upstream support multiple within a single Commit.
2. Did not allow for a byte-to-byte alignment with upstream, due to the above, and its position
set by go-git during encoding time.

In order to mitigate both issues, it will now be handled as part of ExtraHeaders, which
better aligns with upstream.

Signed-off-by: Paulo Gomes <[email protected]>

Signed-off-by: Paulo Gomes <[email protected]>

Tag verification re-encoded from struct fields, so any non-canonical
bytes the signature was computed over (duplicate headers, mutation-only
fields like Signature/SignatureSHA256, etc.) were lost from the payload
and signatures that were valid in upstream Git failed in go-git.

Mirror the approach already used by Commit: retain a reference to the
source plumbing.EncodedObject on Decode, add matchesSource() so
EncodeWithoutSignature can stream the raw bytes (with the inline
trailing PGP block truncated and gpgsig/gpgsig-sha256 headers stripped)
when the struct still matches the source. Mutating struct fields still
falls back to the struct-encoded payload.

Assisted-by: Claude Opus 4.7 <[email protected]>
Signed-off-by: Paulo Gomes <[email protected]>

Align Commit.Verify with upstream's parse_buffer_signed_by_header
(commit.c:1186) and parse_gpg_output rejection (gpg-interface.c:257-269).

The commit scanner now concatenates every gpgsig/gpgsig-sha256 header
into a single signature buffer instead of first-wins, and Commit.Verify
returns the new ErrMultipleSignatures when that buffer carries more
than one armored block.

Assisted-by: Claude Opus 4.7 <[email protected]>
Signed-off-by: Paulo Gomes <[email protected]>

Signed-off-by: Paulo Gomes <[email protected]>

Signed-off-by: Paulo Gomes <[email protected]>

Signed-off-by: Paulo Gomes <[email protected]>

Signed-off-by: Paulo Gomes <[email protected]>

Signed-off-by: Paulo Gomes <[email protected]>

Signed-off-by: Paulo Gomes <[email protected]>