Clear redis cache and precompile assets during update to 6.0. Fix #4874 #4840

As a consequence on the namespaces change from 5.4 to 6.0, after upgrade
lots of links on the dashboard were not working (pointing to projects
without the group part on the url).
Also, after upgrading to 6.0-stable, the network (branch history) function
does not work. The progress bar rotates forever, but the network graph cannot
be generated.

Clearing redis cache fixes those issues.

[#4821] - Fix some missing iids

You should update to 6.0 before proceed with update to higher version.
So its no reason to keep old tasks any more.
It should be still available in 6-0-stable but not in master

Feature: get the diff of a commit via API.

When that branch contain a '/' in the branch name.
Fix for git over HTTP

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add link on realisation api for java client

Add 'hooks' to path blacklist.

bind unicorn listener to 127.0.0.1

Fix GitLab Shell version check

Select files with diff filter for graph log

Fix the `service gitlab restart` command

Postgress db migrate fails.(5.4=>6.0)

People like Karen and myself are confused by the placeholder, it seems like you don't have to change the master value. Also the to and from should be more prominent.

This commit addresses two related issues:
- Sidekiq should shut down completely before we restart it;
- If we restart unicorn with USR2 we need to kill the old master and
  worker processes.

Added assets:precompile in upgrade guide 5.4 -> 6.0