Option to specify branch upon install and update

Problem to solve

For a developer to easily "pick up" another contributor's feature branch with GDK, it would be great if one was able to specify a branch during gdk install and (potentially) gdk update. @nfriend touched on a related issue with #461 (closed).

An enterprise customer/prospect first raised this issue and a staff engineer concurred with the usefulness of this suggestion.

Intended users

Developers, Designers, Product Managers could all benefit from a easy one-liner to spin up a local GL instance in the context of an un-merged feature branch (either to continue development or to evaluate a proposed change).

Further details

https://gitlab.slack.com/archives/CDRLZUVHB/p1571174072041000

https://app.slack.com/client/T02592416/C2Z9A056E/thread/C2Z9A056E-1571175711.039400

Proposal

Example:

gdk install gitlab_repo=https://gitlab.com/<YOUR-NAMESPACE>/gitlab.git branch=<YOUR-FEATURE-BRANCH>

Documentation

Testing

What does success look like, and how can we measure that?

If a user can specify a branch at install and have it persist through update operations, I believe this issue would be successfully addressed.