Also shift logic to directory-based organization instead of a single file with all lists

GitHub has requested we request our usage of shallow clones. As a
result:

- do default to shallow clones on CI
- do not do --shallow-since when setting up new repositories in
  `brew update`
- do not support shallow clones of homebrew-core at all

We may consider in future unshallowing all clones unconditionally.

Don't raise an exception only to catch it.

This was deprecated and removed a while ago.

Don't want to restrict this for all taps otherwise existing ones may
explode.

Fixes #7734

Fixes #7732.

- Use Ruby attribute arguments.
- Fix use of `full_clone` in `cmd/tap` to be on by default.
- Remove unnecessary argument in test.

This makes `Tap` consistent with what the installer is doing.

Generally shallow clones get slower and slower (and more and more
pointless) the more they are fetched so don't make sense for our
use-case.

Keep the option around anyway because it's useful for integration tests.

- Output a message whenever a non-default remote is set.
- Ensure that `CoreTap.instance.install` uses the right remote.

- The `--local` option doesn't exist in early versions of git (~1.7),
  but its behaviour is the default (saving in the current repo's .git
  directory).
- As it's the default across versions, we don't need to specify it
  everywhere.
- Reported in Linuxbrew/install issue 78.

check_coretap_git_origin was not working as intended.
Permit Linuxbrew/homebrew-core as a valid origin.
Factor out check_coretap_git_branch.