- [x] Add support for disabling cache via environment variable => extend Manifest
- [x] Add `lookup` command that returns the cache OID on stdout (exit 0) or exists 1 (and does not
      copy).
- [ ] Is ancestry necessary or is a simple diff sufficient?
- [ ] Add `--verbose,-v` for increased logging levels (overrides RUST_LOG; mention both in help).
- [ ] Add `--no-stdout` and `--no-stderr` to silence outputs intended for machine and human parsing,
      respectively.  Add `--quiet,-q` to combine `--no-stdout` and `--no-stderr`.
- [ ] Replace `env_logger` with `pretty_env_logger`?
- [ ] Check if ancestry and diffing work properly with merges (safe case: fail).
- [ ] Figure out dependency and conflicts w.r.t. `Memora.yml`.  Is `Memora.yml` a necessary implicit
      input dependency?
- [ ] Add CI for Memora itself.
- [ ] Automate releases and builds of `master`.
- [ ] Implement globbing on outputs.
- [ ] Add support for remote caches (at least for write access).  => e.g., extend Manifest with
        ```rust
        /// Host name for write access to the cache directory.
        pub cache_host: Option<String>,
        /// User name for write access to the cache directory.
        pub cache_user: Option<String>,
        ```
- [ ] Implement tests for Git and Cache modules.
- [ ] Extend documentation.
