Turns out the "standard" way of wrapping errors in Go is via
`fmt.Errorf("%w")`, which doesn't require an external package and also allows a
finer control of error sentence formatting.
Also define a handful of utility methods:
- `New(owner, repo)`
- `FullName`: the name slash owner pair
- `FromFullName`: parse the name slash owner pair
- `FromURL`: parse a GitHub.com URL
- `IsSame(r1, r2)`: compare two repositories
- The local git remotes are scanned and resolved to GitHub repositories
- The "base" repo is the first result resolved to its parent repo (if a fork)
- The name of the default branch is read from the base repo
- The "head" repo is the first repo that has push access
