Add Repository#find_branch to speed up branch lookups

See merge request !119

refs DB

See https://gitlab.com/gitlab-org/gitlab-ce/issues/15392#note_14538333

A common call in GitLab is to lookup a single branch, but previously
this was done by calling Repository#branches, which loads all the
branches into memory unnecessarily and causes many filesystem
accesses for each branch. With Repository#find_branch, we can
do a direct lookup for the branch we care about.

Fix broken specs caused by update to gitlab-git-test

Taken from https://gitlab.com/gitlab-org/gitlab_git/merge_requests/118

See merge request !120

Taken from https://gitlab.com/gitlab-org/gitlab_git/merge_requests/118

Remove unneeded call to Repository#root_ref in #log

See merge request !117

From our monitoring data, it seems that Repository#root_ref can be slow
sometimes (probably because it involves iterating over all branches),
and there's no need to have it in the `default_options` hash since
a similar effect is achieved in `actual_ref = options[:ref] || root_ref`
below, and subsequent calls don't need a `:ref` key in the passed
options.

Optimize fetch of the author and committer of a Rugged commit

Closes #26

See merge request !116

We're using Forwardable so we need to require it

See merge request !92

Compare returns an empty collection of commits on nil refs

See merge request !114

Write .gitattributes in binary mode to prevent Rails from converting ASCII-8BIT to UTF-8

This avoids Sidekiq errors in the PostReceive task due to .gitattributes files having ISO-8859 characters, such as:

```
Encoding::UndefinedConversionError: "\xC3" from ASCII-8BIT to UTF-8
```

Closes gitlab-org/gitlab-ce#20647

See merge request !115

Closes gitlab-org/gitlab-ce#20647

Lazy load compare commits

See merge request !113

Clean up local branches after creation to make rspec pass



See merge request !112

Use an exact rubocop-rspec version that's compatible with ruby 2.1

... otherwise it breaks rubocop and rspec builds.

See merge request !110

Add deltas_only option for DiffCollection

See merge request !109

It helps avoiding loading the actual patch (which can consume lots of
memory) when not needed.

Improve performance of a decorated DiffCollection instance

See merge request !108

Add forwardable require to Repository

This class makes use of the Forwardable stdlib but didn't require it
explicitly.

See merge request !107

This class makes use of the Forwardable stdlib but didn't require it
explicitly.

Test against Ruby 2.3 and use caching of gems

See merge request !106

Expose tags git object sha (if it's an annotated tag)

Closes #24

See merge request !104