Xgit.Repository.OnDisk should read config file and configure git_dir accordingly.

[scouten]

• Add label: "TO DO"
• Add label: "good first issue" or "help wanted" if appropriate
• Add link to this issue in a comment near the TO DO comment

Describe What Is Incomplete
The implementation of Xgit.Repository.OnDisk.init/1 isn't very smart about the location of the git directory (typically .git inside the working directory).

Definition of Done

• Can read the config file and configure itself appropriately.
• Can accept a git_dir option and configure itself appropriately (possibly as a bare repo).