Updates specs to use new rails5 format.

The old format:
`get :show, { some: params }, { some: headers }`

The new format:
`get :show, params: { some: params }, headers: { some: headers }`

As we now support getting the merge base for multiple revisions in
gitaly, we can provide this functionality in our API

This adds an endpoint to get the common ancestor of 2 refs from the API.

Repository compare API now allows choosing straight (from..to) or merge-base diff (from...to)

[API] Fix URLs in the 'Link' header for 'GET /projects/:id/repository/contributors' when no value is passed for 'order_by' or 'sort'

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

Allows ordering in GET api/v4/projects/:project_id/repository/contributors
through `order_by` and `sort` params.
The available `order_by` options are: name|email|commits.
The available `sort` options are: asc|desc.

Removed all the unnecessary include of `WaitForAjax` and `ApiHelpers` in the specs.
Removed unnecessary usage of `api:true`

Remove unecessary endpoint from repository, add compatibility endpoints for v3 and several improvements

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

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

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

Restore changes for api spec files

Fix error in rspec Users

Delete extra space Repositories-spec

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

Closes #3311

Signed-off-by: Jeroen van Baarsen <jeroenvanbaarsen@gmail.com>

Create RepoTag Grape entity and present it when doing stuff with tags via API. Update API doc for repositories. Add tag message to tag list page in UI. Update Changelog. Update spec to set .gitconfig identity, required for annotated tags.

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