Similar to https://gitlab.com/gitlab-org/gitlab-ce/issues/63030,
when the commit message in the /resolve_conflicts endpoint contains a
UTF-8 character, the conversion to the Gitaly ASCII-8BIT value may flag
an error. To fix this, we run `force_encoding` on the commit message.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/63062

This allows us to set the encoding of an IO passed without reading it
into memory.

This is useful if we want to stream files into Gitaly. Like we do when
uploading a new file to the repository.

Enables frozen string for the following:

* lib/gitlab/fogbugz_import/**/*.rb
* lib/gitlab/gfm/**/*.rb
* lib/gitlab/git/**/*.rb
* lib/gitlab/gitaly_client/**/*.rb
* lib/gitlab/gitlab_import/**/*.rb
* lib/gitlab/google_code_import/**/*.rb
* lib/gitlab/gpg/**/*.rb
* lib/gitlab/grape_logging/**/*.rb
* lib/gitlab/graphql/**/*.rb
* lib/gitlab/graphs/**/*.rb
* lib/gitlab/hashed_storage/**/*.rb
* lib/gitlab/health_checks/**/*.rb

Partially address gitlab-org/gitlab-ce#47424.

- Add a Gitlab::Git::Conflict::Resolution class to encapsulate
resolution data
- Simplify conflict file collection assembly