Add rake task 'gitlab:update_commit_count'

Starting with migration `20150717130904` commit count is stored in the
database. For existing projects it defaults to `0` and is updated to the
correct value when commits are pushed.

The newly introduced rake task updates the commit count for all projects
which have not been updated yet.

![gitlab-rake-update-commit-count](https://gitlab.com/gitlab-org/gitlab-ce/uploads/4785009e0f3fc4c3199fe65dfb8e60e0/gitlab-rake-update-commit-count.png)

Refs !986, !989, #2040, #2089.

See merge request !1040