Import the gitlab-shell "gitlab projects" functionality into GitLab CE

Copy gitlab-shell/lib/gitlab_projects.rb into gitlab-ce/lib/gitlab/git/gitlab_projects.rb (or so)

---

Old description:

Currently, gitlab-shell uses SIGKILL unconditionally if any of its git subprocesses exceed the user-specified timeout. This can lead to corruption of the repository and is generally a bad idea.

Instead, we should send SIGTERM, wait ~5 seconds, then send SIGKILL if the process hasn't died in that time period. For bonus points, if the gitlab-shell subprocess receives an INT, TERM or similar signal, it should also kill its children before exiting.

There are commands where we allow the subprocess to run without any timeouts at all (e.g., gitlab-projects fork-repository). These should be fixed.