Signed-off-by: NRémy Coutable <remy@rymai.me>

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8091#note_20253762

Signed-off-by: NRémy Coutable <remy@rymai.me>

Signed-off-by: NRémy Coutable <remy@rymai.me>

The reason is that Gitea plan to be GitHub-compatible so it makes sense
to just modify GitHubImport a bit for now, and hopefully we can change
it to GitHubishImport once Gitea is 100%-compatible.
Signed-off-by: NRémy Coutable <remy@rymai.me>

Signed-off-by: NRémy Coutable <remy@rymai.me>

Signed-off-by: NRémy Coutable <remy@rymai.me>

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8088#note_20235080

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/builds/7606797

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8091#note_20222666

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8088#note_20223109

- `raise "string"` raises a `RuntimeError` - no need to be explicit
- Remove top-level comment in the `RevList` class
- Use `%w()` instead of `%w[]`
- Extract an `environment_variables` method to cache `env.slice(*ALLOWED_VARIABLES)`
- Use `start_with?` for env variable validation instead of regex match
- Validation specs for each allowed environment variable were identical. Build them dynamically.
- Minor change to `popen3` expectation.

- Don't define "allowed environment variables" in two places.
- Dispatch to different arities of `Popen.open` without an if/else block.
- Use `described_class` instead of explicitly stating the class name within a
- spec.
- Remove `git_environment_variables_validator_spec` and keep the validation inline.

Previously, we were calling out to `popen` without asserting on the returned
exit-code. Now we raise a `RuntimeError` if the exit code is non-zero.

The list of environment variables in `Gitlab::Git::RevList` need to be validate
to make sure that they don't reference any other project on disk.

This commit mixes in `ActiveModel::Validations` into `Gitlab::Git::RevList`, and
validates that the environment variables are on the level (using a custom
validator class). If the validations fail, the force push is still executed
without any environment variables set.

Add specs for the validation using shared examples.

1. Starting version 2.11, git changed the way the pre-receive flow works.

  - Previously, the new potential objects would be added to the main repo. If the
    pre-receive passes, the new objects stay in the repo but are linked up. If
    the pre-receive fails, the new objects stay orphaned in the repo, and are
    cleaned up during the next `git gc`.

  - In 2.11, the new potential objects are added to a temporary "alternate object
    directory", that git creates for this purpose. If the pre-receive passes, the
    objects from the alternate object directory are migrated to the main repo. If
    the pre-receive fails the alternate object directory is simply deleted.

2. In our workflow, the pre-recieve script (in `gitlab-shell) calls the
   `/allowed` endpoint, which calls out directly to git to perform
   various checks. These direct calls to git do _not_ have the necessary
   environment variables set which allow access to the "alternate object
   directory" (explained above). Therefore these calls to git are not able to
   access any of the new potential objects to be added during this push.

3. We fix this by accepting the relevant environment variables
   (GIT_ALTERNATE_OBJECT_DIRECTORIES, GIT_OBJECT_DIRECTORY) on the
   `/allowed` endpoint, and then include these environment variables while
   calling out to git.

4. This commit includes (whitelisted) these environment variables while making
   the "force push" check. A `Gitlab::Git::RevList` module is extracted to
   prevent `ForcePush` from being littered with these checks.