audit: Use `fetch` with default value prevent NPE

Don’t use `CLI::Cleanup` in `Auditor`.

audit: Add null check on style results

Check version conflicts using linkage.

Instead of refusing to install software preemptively by assuming
multiple linkage to differing versions of the same library we now make
`brew linkage --test` verify that we don't have two versions of the
same library linked at the same time.

This will be considerably more permissive whilst checking the actual
problem that we're worried about.

extract: update docs

Preliminary blurb in documentation about using brew extract to obtain deprecated/deleted/particular versions of formulae.

audit: Show RuboCop offense correction status and count in output

Add new extract dev command to retrieve old versions of formulae

Instead of just blanketing over with monkey-patches like before,
set up monkey-patches as needed, and make sure to clean up after
we're done.

For the impatient ones out there.

Fixes the empty rev string when copying file from HEAD; explicitly
set rev to HEAD in that case.

If the formula to be retrieved was just version-bumped in the most
recent commit (HEAD), we would've ended up grabbing the second-most
recent version.

Instead, if the file already exists in tree at the current commit,
just construct the formula from that to get the version (for naming
purposes) and copy the file outright to its new location.

Fix for Linux make shim

brew.sh: Export HOMEBREW_GIT

Should have been exported like HOMEBREW_CURL.

audit: remove "`bottle` is not defined"

formula_cellar_checks: .dylib and .framework are macOS-specific

manpage: update from latest homebrew-bundle docs.

dev-cmd/bottle: save local_filename to json