This adds possibility to identify erased jobs.

# Conflicts:
#	src/main/java/org/gitlab4j/api/GroupApi.java

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Co-authored-by: Konstantin Hintermayer <konstantin@handig-eekhoorn.at>
Co-authored-by: Jeremie Bresson <jeremie.bresson@unblu.com>

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

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

Fixes #992

Fixes #1006

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

Fixes #1009

Co-authored-by: t.mucs <t.mucs@4flow.de>

Fixes #984

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>

Correct JavaDoc for "protectTag" method to show actual information of GitLab endpoint method.

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

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

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