chore(deps): bump git2 to 0.21.0 to clear RUSTSEC-2026-0183/0184 (#733)

[gustav-fff]

Closes #733

Root cause

Workspace pin git2 = "0.20.2" in Cargo.toml:29 resolves to 0.20.4, which carries two informational = "unsound" advisories (RUSTSEC-2026-0183, RUSTSEC-2026-0184). fff does not call the affected APIs (Remote::list, Blame::blame_buffer) — usage is limited to Repository, StatusOptions, Status*, Version::get — but every downstream consumer of fff-search/fff-grep/fff-query-parser sees the advisories in cargo audit/cargo deny and must add a suppression.

Fix

Bump workspace git2 to 0.21.0. Feature set unchanged (default-features = false, vendored-libgit2). 0.21 changes StatusEntry::path() from Option<&str> to Result<&str, git2::Error>; adjust the two callers in crates/fff-core/src/git.rs:65 and crates/fff-core/tests/fuzz_git_watcher_stress.rs accordingly (drop non-UTF-8 paths, same behavior as before).

Steps to reproduce

Confirm the advisories surface on pre-fix origin/main:

cargo install cargo-audit --locked
git fetch origin main && git checkout origin/main
cargo audit --file Cargo.lock

Expected: two warnings

Crate:     git2
Version:   0.20.4
Warning:   unsound
Title:     Remote::list() passes null pointer to slice::from_raw_parts()
ID:        RUSTSEC-2026-0183

Crate:     git2
Version:   0.20.4
Warning:   unsound
Title:     BlameHunk from Blame::blame_buffer() can build Signatures from null pointers
ID:        RUSTSEC-2026-0184

How verified

cargo update -p git2
# Updating git2 v0.20.4 -> v0.21.0
# Updating libgit2-sys v0.18.3+1.9.2 -> v0.18.7+1.9.6

cargo check --workspace --tests
# Finished `dev` profile [unoptimized + debuginfo] target(s)

cargo test --workspace
# all suites pass: 124 lib tests + integration suites, 0 failed

Diff size: 4 files, +8/-9.

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