Added tests

Added changelog entry

Resolved all issues in code

See #24638

Closes #24680

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7617#note_18763033

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

nothing to merge in merge request page

Also added tests, and moved the slack test to a better location.

This refactors repository caching so it's possible to selectively
refresh certain caches, instead of just expiring and refreshing
everything.

To allow this the various methods that were cached (e.g. "tag_count" and
"readme") use a similar pattern that makes expiring and refreshing
their data much easier.

In this new setup caches are refreshed as follows:

1. After a commit (but before running ProjectCacheWorker) we expire some
   basic caches such as the commit count and repository size.

2. ProjectCacheWorker will recalculate the commit count, repository
   size, then refresh a specific set of caches based on the list of
   files changed in a push payload.

This requires a bunch of changes to the various methods that may be
cached. For one, data should not be cached if a branch used or the
entire repository does not exist. To prevent all these methods from
handling this manually this is taken care of in
Repository#cache_method_output. Some methods still manually check for
the existence of a repository but this result is also cached.

With selective flushing implemented ProjectCacheWorker no longer uses an
exclusive lease for all of its work. Instead this worker only uses a
lease to limit the number of times the repository size is updated as
this is a fairly expensive operation.

Also allow merge request to be merged with skipped pipeline and the
"only allow merge when pipeline is green" feature enabled
Signed-off-by: NRémy Coutable <remy@rymai.me>

Fixes #24677

This reverts commit f913170e.

Initializing Rugged objects is way too expensive just to check if a
repository exists. Even though we cache this data once in a while we
have to refresh this. On GitLab.com we have seen Repository#exists?
taking up to _1 minute_ to complete in the absolute worst case, though
usually it sits around a second or so.

Using File.exist? to instead check if $GIT_DIR/refs exists is a much
faster way of checking if a repository was initialized properly.

This moves the logic of detecting special repository files (e.g. a
README or a Koding configuration file) to a single class:
Gitlab::FileDetector. Moving this logic into a single place allows this
to be re-used more easily.

This commit also changes Repository#gitlab_ci_yaml so that its cached
similar to other data (e.g. the Koding configuration file).