git_repository: define #to_str

Fixes:

    Error: no implicit conversion of GitRepository into String

when doing `brew pr-pull --autosquash`. See [1].

I contemplated implementing only `#to_s`, but we've historically used
these as `Pathname`s, and `Pathname` defines `#to_str`.

[1] https://github.com/Homebrew/homebrew-core/actions/runs/4784638305/jobs/8506369135#step:10:54

Library/Homebrew/git_repository.rb

@@ -109,6 +109,12 @@ class GitRepository
     popen_git("log", "-1", "--pretty=%B", commit, "--", safe: safe, err: :out)&.strip
   end
 
+  sig { returns(String) }
+  def to_str
+    pathname.to_str
+  end
+  alias to_s to_str
+
   private
 
   sig { params(args: T.untyped, safe: T::Boolean, err: T.nilable(Symbol)).returns(T.nilable(String)) }