Update gitlab_git to 5.7

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
 Merge branch 'gitlab_git_5-7' into 'master'

Update gitlab_git to 5.7
Fixed readme page title
Add GFM autocompletion for MergeRequests

Requested
[here](http://feedback.gitlab.com/forums/176466-general/suggestions/5507440-auto-complete-merge-requests-in-gitlab-markdown).
Merge pull request #6475 from tsigo/merge-request-autocomplete

Add GFM autocompletion for MergeRequests
Merge pull request #6473 from veonik/fix-readme-title

Fixed readme title
Added part about the help to markdown