It was my mistake to establish several tracking lines for tracking
upstream changes - e.g. in

    61544d87    (gitlab: Import nginx http configuration from omnibus-gitlab)

we started not from

    6fd7b987    (gitlab: Import gitlab-ce & gitlab-shell configs from omnibus-gitlab)

-- the first upstream tracking commit on its own branch -- but from

    4c127fdd    (gitlab: Setup sidekiq service)

i.e. from after some changes which already tweaked upstream
configuration files.

This makes updating gitlab more work than necessary: instead of
switching to upstream branch only once, importing all files, and
then switching back to master and merging upstream changes only once, we
currently have to do that operation 3 times:

    - for main gitlab settings,
    - for nginx settings, and
    - for gitconfig settings

which is not convenient and wastes our time.

So establish a proper 1 branch for tracking upstream configs:

Here we cherry-pick the following commits

    61544d87    (gitlab: Import nginx http configuration from omnibus-gitlab)
    d17f1f5f    (gitlab: Sync nginx http configuration from omnibus gitlab)

    8f945bd2    (gitlab: Import gitconfig from omnibus-gitlab)
    e8461571    (gitlab: Sync gitconfig settings from omnibus-gitlab)

and later we'll be updating upstream files on a branch starting from
this commit and containing upstream changes only.

/cc @kazuhiko, @jerome