Support for installing in "bare" with depth=1 and no .git folder

Tools like `npm` have a special command for use in CI environments.

It would be great if gitman had something like this as well.
It should install dependencies and remove the `.git` dirs so that we don't unnecessarily bloat our builds.

Start with gitman’s install command and the dependency-installation flow, then trace where Git checkouts are created. Done means a bare installation supports depth=1 and removes .git directories so CI builds do not retain repository metadata; verify the behavior in a CI-like installation.

Written by the indexing model from the issue text.