feat: add reflog feature with tests

[racgoo]

Summary

• Add 'reflog' feature with tests.

Changes

• Implement reflog-like methods in 'Repository'
• Add internal reflog features with 'ReflogEntry'
• Add test code

Question

@seokju-na I have a small question regarding the error-handling pattern used here (also seen in 'tree.rs' and 'stash.rs').
In 'reflog.get' (reflog.rs:228), the error created by 'ok_or(Error::new(...))' seems to be discarded immediately because the result is converted with '.ok()', collapsing all errors into 'None'.
Since I couldn't find a better approach myself, I wanted to ask whether there is a more idiomatic way to express this without constructing an unused error.

Closes #171

[vercel]

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