Bump Gitaly version to v1.12.0
See https://gitlab.com/gitlab-org/gitaly/blob/master/CHANGELOG.md.
This fixes two main issues:
- Incorrect tree entries retrieved with directories that have curly braces: https://gitlab.com/gitlab-org/gitlab-ce/issues/46261
- Wildcard protected branches not working with remote mirrors: gitaly#1435 (closed)