git-commit: introduce git-commit crate

Porting the commit code that is found in heartwood[0]. It represents a
parsed git commit object -- sans the id for the commit itself.

[0]: https://github.com/radicle-dev/heartwood/blob/16b22d9178fa6a1aaf2ecb01ec97e067399c5286/radicle/src/git.rs#L392-L575

Signed-off-by: Fintan Halpenny <fintan.halpenny@gmail.com>
X-Clacks-Overhead: GNU Terry Pratchett

git-commit/t/src/integration.rs

@@ -0,0 +1,28 @@
+use std::io;
+
+use git_commit::Commit;
+use test_helpers::tempdir::WithTmpDir;
+
+#[test]
+fn valid_commits() {
+    let radicle_git = format!(
+        "file://{}",
+        git2::Repository::discover(".").unwrap().path().display()
+    );
+    let repo = WithTmpDir::new(|path| {
+        let repo = git2::Repository::clone(&radicle_git, path)
+            .map_err(|err| io::Error::new(io::ErrorKind::Other, err))?;
+        Ok::<_, io::Error>(repo)
+    })
+    .unwrap();
+
+    let mut walk = repo.revwalk().unwrap();
+    walk.push_head().unwrap();
+
+    // take the first 20 commits and make sure we can parse them
+    for oid in walk.take(20) {
+        let oid = oid.unwrap();
+        let commit = Commit::read(&repo, oid);
+        assert!(commit.is_ok(), "Oid: {}, Error: {:?}", oid, commit)
+    }
+}