Accept environment variables from the `pre-receive` script.
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.
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