info: avoid broken GitHub URL for removed formulae

When a formula has been removed from its tap but is still installed,
`FromKegLoader` loads it from the keg's `.brew/<name>.rb` with the tap
still set from the install receipt. `github_info` then built a "From:"
URL by joining the tap's GitHub remote with
`formula.path.relative_path_from(tap.path)`, producing a path full of
`../` components, e.g. for `terraform`:

  https://github.com/Homebrew/homebrew-core/blob/HEAD/../../../../opt/terraform/.brew/terraform.rb

Fall back to the formula's local path in that case, matching the
existing behaviour for formulae with no tap.