feat: Use Git CLI for cloning repositories by default

[siketyan]

The Git backend ghr uses was libgit2 in v0.1 releases.
However, in this strategy, we could not clone repository in an authenticated context (e.g. cloning your private repo, or your organisation's repo).

Now ghr uses Git CLI (git command simply) by default.
You can also change this behaviour by modifying config.toml file as follows:

[git]
stategy.clone = "git2"  # `cli` or `git2`

Have fun!