feat: expose git blame via Repository.blame()

[shenxianpeng]

Summary

Wraps gix's blame engine as Repository.blame(path, rev=None), the first
piece of the feature-gated gix surface this binding has reached for.

>>> repo = gitoxide.open(".")
>>> for hunk in repo.blame("README.md")[:3]:
...     print(f"{hunk.start_line}-{hunk.end_line}\t{hunk.short_id}")
1-2     b55a5e0
3-3     f6a6ddd
4-5     295428e

What's here

• Cargo.toml — enables the blame feature of gix (which pulls in
  blob-diff → attributes).
• Repository.blame(path, rev=None) — rev defaults to HEAD.
• BlameHunk — start_line, end_line, line_count, orig_start_line,
  commit_id, short_id, plus a __repr__.
• Line numbers converted to 1-based inclusive, matching git blame. gix
  counts from 0, so passing its values straight through would have been off by
  one in every hunk.
• 4 tests against the existing sample_repo fixture, including the exact
  commit attribution for a two-line file, blame at an older revision, and the
  error path for a missing file.
• README — quick-start example, API reference, and blame moved from
  "not wrapped yet" to "wrapped".

Cost

Measured on macOS arm64, release + thin LTO:

	before	after
wheel	1.51 MB	1.99 MB (+31.7%)
cold build	26s	~38s
transitive crates	113	125

The twelve new crates (gix-blame, gix-imara-diff, gix-filter,
gix-worktree, gix-attributes, gix-dir, …) are all pure Rust — wheels
stay self-contained and still need no C toolchain.

Worth knowing: enabling the feature without calling it only costs ~1%,
because LTO strips the unreachable code. The +31.7% above is the real,
measured cost with the binding actually wired up.

One caveat, reflected in the README

blob-diff brings in gix-filter, which will execute a configured
clean/smudge filter (Git LFS, for example) when blaming such a file — the
same thing git does. Nothing shells out to the git CLI, so the comparison
against GitPython still holds, but the blanket "no subprocess" line in the
README now carries a footnote.

Verification

pytest -q → 20 passed (16 existing + 4 new). cargo fmt --check and
cargo clippy --all-targets -- -D warnings both clean. The README example is
copied from a real run against this repository.

Not included

repo.is_dirty() was floated alongside this, but it lives behind gix's
status feature, not blame — and status drags in dirwalk + index
on top of blob-diff. Separate change, separate cost.