Fixes #992

Co-authored-by: Jan Henrik Wiesner <jan.wiesner@gebit.de>
Co-authored-by: Jeremie Bresson <jeremie.bresson@unblu.com>

Changes MergeRequestApi:

- added methods "getDiffs()" for GitLab Endpoint:
GET /projects/:id/merge_requests/:merge_request_iid/diffs

- add methods "getDiffVersions()" for GitLab Endpoint:
GET /projects/:id/merge_requests/:merge_request_iid/versions

- add method "getDiffVersion()" and "getOptionalDiffVersion()" for GitLab Endpoint:
GET /projects/:id/merge_requests/:merge_request_iid/versions/:version_id

- deprecate "getMergeRequestDiffs()" and "getMergeRequestDiff()"

Fixes #993

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Co-authored-by: Jeremie Bresson <jeremie.bresson@unblu.com>

* Add methods to create child epics

* Add methods to manage related epics

* UserApi implement createPersonalAccessToken

* adds missing scopes
* factors out common logic

* Fix integration tests to run with Gitlab "12.9.2-ce.0"

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Co-authored-by: Malte Meister <malte@labs.ws>

* Add missing attributes in EventMergeRequest

* Fix duration and tests

* Fix Epic Issue link association

Fixes #729

* Change return type

* Add repository submodules api

Fixes #921

* Fix unit test

* Added ReleaseLinks API (#639)

* Deprecate the "external" attribute in Link

* Add tests

* Rename to ReleaseLinksApi and add the getter in GitLabApi

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Co-authored-by: Jeremie Bresson <jeremie.bresson@unblu.com>

Add iterationTitle in IssueFilter for filtration issues
by this field

Co-authored-by: Усачев Дмитрий Дмитриевич <92usachevdd@region.cbr.ru>

* feat(scheduled-pipeline): makes scheduled pipelines runnable

* Update src/main/java/org/gitlab4j/api/PipelineApi.java

Co-authored-by: Jérémie Bresson <dev@jmini.fr>

* fix(scheduled-pipeline): change post()-call

* Update src/main/java/org/gitlab4j/api/PipelineApi.java

Co-authored-by: Jérémie Bresson <dev@jmini.fr>

* Add missing import

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Co-authored-by: Jérémie Bresson <dev@jmini.fr>
Co-authored-by: Jeremie Bresson <jeremie.bresson@unblu.com>
Co-authored-by: Ruben Vitt <dev@rubeen.one>

* Add ExternalStatusCheckApi

Fixes #919

* Add ExternalStatusCheckEvent

* Fix javadoc

Fixed de-serialization of MembershipSourceType by letting the JacksonJsonEnumHelper know that the first letter should be capitalized

This makes it possible to stream data coming
from another server straight into the POST request to Gitlab,
saving an extra, expensive, and useless IO operation on disk.

Fix #769

A 'scope' attribute was added to the Events API that allows retrieval of all events for a user across all the projects that user has access to.  This change adds support for this new attribute.

Ensuring test SystemHooks are removed if tests fail, adding commentary/todo to address GitLab issue at future date