Changed sudo by su -c

The sudo command needs a tty to run. If you put this file on a CentOS 6, for example, it will not run in the host startup process. Instead, the 'su -c' runs the command apart if it has or not a tty.

I tested this change in a fresh install of gitlab on a CentOS 6.5 with successful.
Fixed attachment identifier displaying underneath note text
Also expose username for the /discover endpoint

Fixes #6328
Added executable file permission to binstubs
Update merge_request_observer_spec.rb
Bumped the gitlab_git gem version
Added the contribution guide notice

This is shown at the creation of new issues and new merge requests, when
the repository has a contribution guide file.
Add button for toggling inline comments in diff view.

This is useful when there are many comments and they're becoming a distraction when trying to read the diff.
Remove dir prefix and suffix from tests inside dir.
Added Piwik support
Don't send an email for "mentioned in" notes

Currently, an email is sent every time a mentionable is referenced
by an issue, a commit or a merge request: if I comment "This MR is
related to #5", watchers get one email for the comment, and another
one stating "Issue #5 was mentioned by issue #13".

This is annoying — but the biggest issue is when pushing an existing
branch. Every issue referenced by commit messages in this branch will
get a new mention (which is fine), and dozens of emails will be sent
for all these new mentions (which is not).

This commit fixes the spam by avoiding to send an email when a new
mention is created. In most cases the email notification for the
mentioner is enough.
Add reaping_frequency for MySQL production DB

This helped avoid ActiveRecord::ConnectionTimeoutError errors for
Sidekiq on GitLab Cloud.
Change Gitlab::Popen to use arrays for commands
Make the Gitlab::Popen path argument optional
Make it possible to call Gitlab::Popen.popen
Adapt use of Gitlab::Popen to new style
Fixed typo in comments of apllication.scss
Better redirect for edit blod from MergeRequest

If you cancel edit you will be redirected back to merge request
If you submit changes you will be redirected back to merge request

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

Better redirect for edit blobs from MergeRequest
 Merge branch 'gitlab_popen_array' into 'master'

Change Gitlab::Popen to only accept arrays as commands