test: add git manager quality gate

[dinakars777]

Summary

Adds a real quality gate for git-time-machine by making CI fail on formatting and clippy warnings, then backs the core Git wrapper with regression tests that create temporary repositories and exercise real Git commands.

The new tests cover reflog parsing order, dirty-worktree detection, diff stat generation, and invalid hash handling. This gives future recovery and restore changes a safer base than the previous zero-test suite.

Verification

• cargo fmt --check
• cargo clippy -- -D warnings
• cargo test
• cargo build
• git diff --check