Added assets:precompile in upgrade guide 5.4 -> 6.0

Fix bug in ace-src-noconflict with Relative url.

Render not found when failing to look for commit

1. Replace params key 'q' with 'extended_sha1'. A extended SHA1 syntax is explained in 'man gitrevisions'.
2. Change the placeholder of looking for commit.
3. Change the label of ref filter.

fix checkout tag name

If we doesn't use a relative url we have this:
<pre>gon.relative_url_root=""</pre>
Else <pre>gon.relative_url_root="/gitlab"</pre>

Gitlab::Application.config.assets.prefix contains:
<pre>/assets</pre>

Update to gitlab-shell 1.7.0 during 5.4-6.0 update

Remove the dependancy of grit from class Network::Graph