Cleanup fetch and mirror methods

In preparation for https://gitlab.com/gitlab-org/gitlab-ce/issues/39252.

CE counterpart: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15424

Before, we had:

• Project#fetch_mirror
• Repository#fetch_remote
• Repository#fetch_mirror
• Repository#fetch_upstream
• Repository#fetch_geo_mirror
• Gitlab::Git::Repository#fetch
• Repository#set_remote_as_mirror
• Repository#set_import_remote_as_mirror

Major changes:

1. Added Repository#fetch_as_mirror, does roughly what Gitlab::Git::Repository#fetch_mirror did but using Repository#fetch_remote
2. Gitlab::Git::Repository#fetch_mirror: renamed to Gitlab::Git::Repository#fetch_as_mirror_without_shell to prevent confusion
3. Repository#fetch_geo_mirror: removed, call site now uses Repository#fetch_as_mirror
4. Gitlab::Git::Repository#fetch: renamed to Gitlab::Git::Repository#fetch_remote_without_shell to prevent confusion
5. Repository#set_import_remote_as_mirror: absorbed into Repository#set_remote_as_mirror