Add example showing retrieving of latest commit for a file

[DanielEScherzer]

Closes #588
Closes #943

[DanielEScherzer]

Testing locally:

root@1976037f2eef:/rust/git2-rs# cargo run --example file-latest-commit -- --path examples/cat-file.rs
warning: libgit2-sys@0.18.3+1.9.2: failed to probe system libgit2:
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.30s
     Running `target/debug/examples/file-latest-commit --path examples/cat-file.rs`
Most recent commit modifying examples/cat-file.rs on default branch: a75e007a6eb1493e9ea487b12fe4ea7a7917ffc1

root@1976037f2eef:/rust/git2-rs# cargo run --example file-latest-commit -- --path examples/cat-file.rs --branch return-results
warning: libgit2-sys@0.18.3+1.9.2: failed to probe system libgit2:
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.37s
     Running `target/debug/examples/file-latest-commit --path examples/cat-file.rs --branch return-results`
Most recent commit modifying examples/cat-file.rs on branch return-results: d791501c0e5c21c56bf264414eef9b5f91699a62

For a75e007 and d791501

[weihanglo]

Thanks!

View changes since this review

[weihanglo]

nit: eprintln! stderr for human. stdout for machine programmatic use case

View changes since the review

[DanielEScherzer]

The overall main() uses println based on the other examples, and I figured I should be consistent with that

[weihanglo]

Yeah I got that hence merged directly