Leaving out the -I option in "git grep" would cause an Error 500 because the
resulting line would include "Binary file X matches"

Gitlab::Git::Repository#branch_count is a tad faster than the previous
setup. See gitlab-org/gitlab_git!62 for more information.

This also enable a project creator to add themselves as a master of the
project.

This greatly improves the performance of Project#external_issue_tracker
by moving most of the fields queried in Ruby to the database and letting
the database handle all logic. Prior to this change the process of
finding an external issue tracker was along the lines of the following:

1. Load all project services into memory.
2. Reduce the list to only services where "issue_tracker?" returns true
3. Reduce the list from step 2 to service where "default?" returns false
4. Find the first service where "activated?" returns true

This has to two big problems:

1. Loading all services into memory only to reduce the list down to a
   single item later on is a waste of memory (and slow timing wise).
2. Calling Array#select followed by Array#reject followed by Array#find
   allocates extra objects when this really isn't needed.

To work around this the following service fields have been moved to the
database (instead of being hardcoded):

* category
* default

This in turn means we can get the external issue tracker using the
following query:

    SELECT *
    FROM services
    WHERE active IS TRUE
    AND default IS FALSE
    AND category = 'issue_tracker'
    AND project_id = XXX
    LIMIT 1

This coupled with memoizing the result (just as before this commit)
greatly reduces the time it takes for Project#external_issue_tracker to
complete. The exact reduction depends on one's environment, but locally
the execution time is reduced from roughly 230 ms to only 2 ms (= a
reduction of almost 180x).

Fixes gitlab-org/gitlab-ce#10771

Previously it was "user has already been taken", when really we were
saying the user has already been reported.

Also fix a few places where page_404 should be render_404.

* Use commit objects instead of IDs when generating diffs
* Use proper references when generating MR's source and target
* Update broken specs

This allows us to implement artifacts passing: runner will download artifacts from all prior builds

Since GitHub doesn’t apply these constraints to theirs wiki slug
allowing characters like `,`, `:`, `*`, etc, we need to relax our
constraints or some wiki pages will not be available after they were
imported.

For an example the Devise project have a wiki page with the following
slug: “How To: Add sign_in, sign_out, and sign_up links to your layout
template”

In this version we will support only relative paths in artifacts
metadata. Support for absolute paths will be introduced later.

Artifacts metadata field will be used to store a filename of gzipped
file containing metadata definition for given artifacts archive.

This is needed because of backward compatibility. Previously artifacts
archive had `.tar.gz` format, but artifacts browser requires ZIP format
now.

`artifacts?` method checks if artifacts archive is available.