Renaming default branch leads to unreachable project settings page
## Summary
After renaming the default branch I can't open the project settings page anymore.
## Steps to reproduce
* You need a repository with some commits and one default branch - I'll call it _A_ in this example
* Open a terminal and go to the path of the repo in your gitlab installation, i.e.`/home/git/repositories/$namespace/$repo.git/`
* Rename the branch to _B_ by the following command: `git branch -m A B`
* Open GitLab in your browser and navigate to the settings page of the project (i.e. `https://gitlab/$namespace/$repo/edit`)
* Error 500 occurs
## Expected behavior
The page of project settings occurs. I am notified that the default branch does not exist and can now change the default branch to `B`.
## Relevant logs and/or screenshots
I think this shows the matching traceback:
```
Started GET "/uib/blocker/edit" for 127.0.0.1 at 2016-06-27 16:07:41 +0200
Processing by ProjectsController#edit as HTML
Parameters: {"namespace_id"=>"uib", "id"=>"blocker"}
Completed 500 Internal Server Error in 108ms (ActiveRecord: 2.5ms)
ActionView::Template::Error (undefined method `sha' for nil:NilClass):
1: %fieldset.builds-feature
2: %h5.prepend-top-0
3: Builds
4: - unless @repository.gitlab_ci_yml
5: .form-group
6: %p Builds need to be configured before you can begin using Continuous Integration.
7: = link_to 'Get started with Builds', help_page_path('ci/quick_start
', 'README'), class: 'btn btn-info'
app/models/repository.rb:525:in `head_tree'
app/models/repository.rb:531:in `tree'
app/models/repository.rb:511:in `gitlab_ci_yml'
app/views/projects/_builds_settings.html.haml:4:in `_app_views_projects__builds_settings_html_haml__2780650300404000149_70213157900520'
app/views/projects/edit.html.haml:89:in `block in _app_views_projects_edit_html_haml___2081903954413398376_70213163819340'
app/views/projects/edit.html.haml:7:in `_app_views_projects_edit_html_haml___2081903954413398376_70213163819340'
app/controllers/projects_controller.rb:24:in `edit'
lib/gitlab/middleware/go.rb:16:in `call'
```
## Output of checks
### Results of GitLab Application Check
```
Checking GitLab Shell ...
GitLab Shell version >= 3.0.0 ? ... OK (3.0.0)
Repo base directory exists? ... yes
Repo base directory is a symlink? ... no
Repo base owned by git:git? ... yes
Repo base access is drwxrws---? ... yes
hooks directories in repos are links: ...
6/3 ... ok
6/4 ... ok
6/6 ... ok
6/7 ... ok
2/8 ... ok
6/9 ... ok
6/12 ... ok
2/14 ... ok
2/15 ... ok
2/16 ... ok
2/17 ... ok
6/18 ... ok
2/20 ... ok
2/21 ... ok
2/23 ... ok
6/24 ... ok
2/28 ... ok
5/29 ... ok
11/31 ... ok
6/34 ... ok
6/35 ... ok
6/38 ... ok
6/39 ... ok
7/42 ... ok
2/43 ... ok
6/44 ... ok
5/45 ... ok
5/46 ... ok
6/47 ... ok
6/48 ... ok
5/50 ... ok
2/51 ... ok
6/52 ... ok
6/54 ... ok
5/55 ... ok
6/56 ... ok
2/63 ... ok
18/64 ... ok
18/65 ... ok
5/67 ... ok
5/68 ... ok
6/69 ... ok
7/70 ... ok
2/71 ... ok
18/72 ... ok
6/73 ... ok
6/78 ... ok
6/79 ... ok
6/80 ... ok
6/81 ... ok
18/83 ... ok
18/84 ... ok
2/85 ... ok
18/86 ... repository is empty
6/87 ... ok
6/88 ... ok
7/89 ... ok
6/90 ... ok
5/91 ... ok
6/93 ... ok
6/94 ... ok
6/95 ... repository is empty
18/96 ... ok
6/97 ... ok
6/98 ... ok
6/99 ... ok
6/100 ... ok
2/101 ... ok
2/103 ... ok
2/105 ... ok
6/106 ... ok
6/107 ... ok
6/114 ... ok
18/115 ... ok
Running /home/git/gitlab-shell/bin/check
Check GitLab API access: OK
Check directories and files:
/home/git/repositories/: OK
/home/git/.ssh/authorized_keys: OK
Send ping to redis server: gitlab-shell self-check successful
Checking GitLab Shell ... Finished
Checking Sidekiq ...
Running? ... yes
Number of Sidekiq processes ... 1
Checking Sidekiq ... Finished
Checking Reply by email ...
Reply by email is disabled in config/gitlab.yml
Checking Reply by email ... Finished
Checking LDAP ...
LDAP users with access to your GitLab server (only showing the first 100 results)
[snip]
Checking LDAP ... Finished
Checking GitLab ...
Git configured with autocrlf=input? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config outdated? ... no
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory setup correctly? ... yes
Init script exists? ... yes
Init script up-to-date? ... yes
projects have namespace: ...
6/3 ... yes
6/4 ... yes
6/6 ... yes
6/7 ... yes
2/8 ... yes
6/9 ... yes
6/12 ... yes
2/14 ... yes
2/15 ... yes
2/16 ... yes
2/17 ... yes
6/18 ... yes
2/20 ... yes
2/21 ... yes
2/23 ... yes
6/24 ... yes
2/28 ... yes
5/29 ... yes
11/31 ... yes
6/34 ... yes
6/35 ... yes
6/38 ... yes
6/39 ... yes
7/42 ... yes
2/43 ... yes
6/44 ... yes
5/45 ... yes
5/46 ... yes
6/47 ... yes
6/48 ... yes
5/50 ... yes
2/51 ... yes
6/52 ... yes
6/54 ... yes
5/55 ... yes
6/56 ... yes
2/63 ... yes
18/64 ... yes
18/65 ... yes
5/67 ... yes
5/68 ... yes
6/69 ... yes
7/70 ... yes
2/71 ... yes
18/72 ... yes
6/73 ... yes
6/78 ... yes
6/79 ... yes
6/80 ... yes
6/81 ... yes
18/83 ... yes
18/84 ... yes
2/85 ... yes
18/86 ... yes
6/87 ... yes
6/88 ... yes
7/89 ... yes
6/90 ... yes
5/91 ... yes
6/93 ... yes
6/94 ... yes
6/95 ... yes
18/96 ... yes
6/97 ... yes
6/98 ... yes
6/99 ... yes
6/100 ... yes
2/101 ... yes
2/103 ... yes
2/105 ... yes
6/106 ... yes
6/107 ... yes
6/114 ... yes
18/115 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.1.0 ? ... yes (2.2.4)
Your git bin path is "/usr/local/bin/git"
Git version >= 2.7.3 ? ... yes (2.7.4)
Active users: 14
Checking GitLab ... Finished
```
### Results of GitLab Environment Info
```
System information
System: Ubuntu 14.04
Current User: git
Using RVM: no
Ruby Version: 2.2.4p230
Gem Version: 2.4.5.1
Bundler Version:1.11.2
Rake Version: 10.5.0
Sidekiq Version:4.1.2
GitLab information
Version: 8.9.1
Revision: c6da519
Directory: /home/git/gitlab
DB Adapter: mysql2
URL: https://gitlab.uib.local
HTTP Clone URL: https://gitlab.uib.local/some-group/some-project.git
SSH Clone URL: git@gitlab.uib.local:some-group/some-project.git
Using LDAP: yes
Using Omniauth: no
GitLab Shell
Version: 3.0.0
Repositories: /home/git/repositories/
Hooks: /home/git/gitlab-shell/hooks/
Git: /usr/local/bin/git
```
issue