Switch directory ownership to git ls-files

This switches away from using a glob match on '**' within
`DirectoryOwnership` matcher in favour of using `git ls-files`. This
provides a significant speed improvement for large repositories for a
few reasons.

The primary reason is that we're only searching through tracked files
and avoiding potentially large directores, such as `node_modules`.

The major intended side effect of this is that it only is dealing with
tracked files rather than all files.

I'm opening the PR for further discussion and refinement to see if this
is worth considering.

Speed comparison for me locally:

Before:
```
❯ time bin/codeownership validate

________________________________________________________
Executed in   16.72 secs    fish           external
   usr time    3.08 secs    0.10 millis    3.08 secs
   sys time   10.70 secs    1.59 millis   10.70 secs
```

After:
```
❯ time bin/codeownership validate

________________________________________________________
Executed in   10.67 secs    fish           external
   usr time    2.90 secs  111.00 micros    2.90 secs
   sys time    8.65 secs  741.00 micros    8.65 secs
```

Author: gmacdougall

lib/code_ownership/private/ownership_mappers/directory_ownership.rb

@@ -41,12 +41,10 @@ def update_cache(cache, files)
             returns(T::Hash[String, ::CodeTeams::Team])
         end
         def globs_to_owner(files)
-          # The T.unsafe is because the upstream RBI is wrong for Pathname.glob
-          T
-            .unsafe(Pathname)
-            .glob(File.join('**/', CODEOWNERS_DIRECTORY_FILE_NAME))
-            .map(&:cleanpath)
-            .each_with_object({}) do |pathname, res|
+          `git ls-files **/#{CODEOWNERS_DIRECTORY_FILE_NAME}`
+            .each_line
+            .each_with_object({}) do |fname, res|
+            pathname = Pathname.new(fname.strip).cleanpath
             owner = owner_for_codeowners_file(pathname)
             res[pathname.dirname.cleanpath.join('**/**').to_s] = owner
           end