Change `commits_by_message` to not mutate string arguments

The following discussion from !33216 (merged) should be addressed:

• @ali-gitlab started a discussion: (+2 comments)

  + is used here to unfreeze the string literal. Without this, the test fails with:

  FrozenError: can't modify frozen String
  
    0) Gitlab::ProjectSearchResults commit search by commit message when repository_ref is provided filters the search by ref
       Failure/Error: revision: revision.to_s.force_encoding(Encoding::ASCII_8BIT),
  
       FrozenError:
         can't modify frozen String
       # ./lib/gitlab/gitaly_client/commit_service.rb:249:in `force_encoding'
       # ./lib/gitlab/gitaly_client/commit_service.rb:249:in `commits_by_message'