Add instructions to install Git from source

This is needed for Ubuntu 12.04, which only provides Git 1.7.9.
Edit config/gitlab.yml when compiling git
Divide Git installation instructions in two
Include full dependencies for Git compilation
Clarify that Git 1.8.x is OK
Add a combined update doc from 5.1 to 6.0
Fix for issue #5431
Merge branch 'install_git_from_source' of /home/git/repositories/gitlab/gitlabhq
Merge pull request #5436 from Seed-Of-Hate/master

Fix for issue #5431
Restore sidekiq rake tasks
Report application status through init exit code
Merge branch 'fix_sidekiq_rake' of /home/git/repositories/gitlab/gitlabhq
Update installation.md

Updated instructions to include proper stable. Otherwise they fail due to Rack extensions required below.
Reset button/link on the reset password page. Fixes #5418
Merge pull request #5448 from flewid/patch-1

Update installation.md
Merge pull request #5453 from perryfaro/password-page

Reset buttons/links on the reset password page
Better font size for projects list on dashboard
rounded avatars
Reduce font-size for event-title
Minor restyle for event filter