I didn't initially use GitHubRepo because it had the weird error
wrapping. I poked around and I think this function is leftover from a
vestigial GitHubRepoID function that used to be more single-purpose. I
took out the weird error handling so it could be reused and then used it
from the new GitHubRepoExists.
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
