Closes #14961

GitHub Pull Requests importer handle with the repository while
importing data, we need to make sure that the cached values are valid.

Closes #13957

This will help us to avoid posible merge conflicts when merging
gitlab-ce to gitlab-ee

Closes #14659

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

Closes #14580

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

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

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

This caches the output of Repository#exists? in Redis while making sure
it's flushed properly when creating new repositories, deleting them,
etc.

For the ProjectWiki tests to work I had to make ProjectWiki#create_repo!
public as testing private methods in RSpec is a bit of a pain.

When calling Repository#build_cache we _don't_ want to build the
diverging commit count cache as doing so can be _very_ slow for
repositories with lots of branches. Because these caches are built
whenever needed (= when actually viewing the list of branches in the web
UI) we can safely remove this code from
Repository#build_cache.

This method is not used or tested anywhere.

The avatar logic has been moved from Project to Repository as this makes
caching easier. The logic itself in turn has been changed so that the
logo file names are cached in Redis. This cache is flushed upon pushing
a commit but _only_ if:

1. The commit was pushed to the default branch
2. The commit actually changes any of the logo files

If no branch or commit is given the cache is flushed anyway, this
ensures that calling Repository#expire_cache without any arguments still
flushes the avatar cache (e.g. this is used when removing a project).

Fixes gitlab-org/gitlab-ce#14363

This ensures the right caches are flushed when adding a branch via the
UI, instead of only flushing this one random cache.

This ensures Repository#add_tag calls Repository#before_push_tag instead
of just 1 random cache expiration method.

This ensures that Repository#rm_branch calls
before_remove_branch/after_remove_branch instead of just 1 random cache
expiration method.

The methods used for this are Repository#tag_count and
Repository#branch_count which cache their output in Redis as well as
memoizing it in an instance variable. Both methods have a corresponding
methods/hooks to flush the caches at the right time.

This is only needed when calling the "expire_cache" method as the other
expiration methods don't depend on Git data.

This keeps all the cache expiration code in a single file/class instead
of spreading it all across the codebase.

This ensures that the emptiness cache (used for Repository#empty? and
Repository#has_visible_content?) is flushed after comitting changes
(using the web editor, API or Git) for new repositories. Once a
repository is no longer empty there's no need to explicitly flush the
cache for Repository#empty?. The cache for
Repository#has_visible_content? in turn is already flushed whenever
needed.

Fixes gitlab-org/gitlab-ce#13387

* Now checking if a commit is already reverted or there is a conflict is
  much more faster.
* No longer need to create a new branch.