Instead of flushing the behind/ahead counts for all branches upon every
push we now only flush the cache of branches that actually need to have
these statistics recalculated. There are now basically 2 scenarios and
their effects:

1. A user pushes a commit to the default branch, this results in the
   cache being flushed for all branches.
2. A user pushes to a non default branch, this results in _only_ the
   cache for that branch being flushed.

The existing code (Repository#expire_cache) remains backwards compatible
with the previous behaviour, the new behaviour is only applied when a
branch name is passed as an argument. This ensures that when for example
a project is deleted the cache for all branches is flushed.

This caches the output of the following methods:

* Repository#empty?
* Repository#has_visible_content?
* Repository#root_ref

The cache for Repository#has_visible_content? is flushed whenever a
commit is pushed to a new branch or an existing branch is removed.
The cache for Repository#root_ref is only flushed whenever a user
changes the default branch of a project. The cache for Repository#empty?
is never explicitly flushed as there's no need for it.

- Enable CI by default for all new projects

If a branch is deleted with an open merge request, amended offline, and then pushed again,
GitLab doesn't bother to update the merge request even though the last commit ID and/or
code may have changed. This MR ensures that each push will update any relevant merge
requests and adds a system note if this happens as well.

Closes #2926

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>

I also revamped the tests on "closing reference commits" (= "Fixes #xxxx" for example).
Now, there are two different contexts:
- when the commits with "closing reference" are pushed to the default branch,
- when the commits with "closing reference" are pushed to a non-default branch.

Closes gitlab-org/gitlab-ce#2190.

This reverts commit 5daf44b7, reversing
changes made to 2f706fbd.

This reverts commit 435f680b.

This reverts commit 957e849f, reversing
changes made to 6b9dbe9f.
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>

Closes #1700
Closes #1720

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

In an empty repository, pushing a new branch not called "master" would
leave HEAD in an unknown state, causing ambiguity if another branch
were pushed. This could in turn cause a new protected branch to be
created and cause the default branch to change.

* Closes #1561
* Closes #1576
* Closes https://github.com/gitlabhq/gitlabhq/issues/8883

This reverts commit 07f9a3f9, reversing
changes made to 48036751.

Reverted this because the data is used in hooks as well.

Remove unnecessary fetch of commit messages for initial push. This will reduce the memory usage significantly.

This commit disables process_commit_messages()
for the initial push to the default branch.
This fixes the mass SQL statements (~500000) that were executed during
the initial push of the linux kernel for example.

1) Adds a DB migration for all services to toggle on push, issue, and merge events.

2) Upon an issue or merge request event, fire service hooks.

3) Slack service supports custom messages for each of these events. Other services
not supported at the moment.

4) Label merge request hooks with their corresponding actions.

Also execute GitLab CI on creating tag via UI

and more readable.

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