Being able to target a particular git SHA version for testing

[chrisdenman]

Is your feature request related to a problem? Please describe.
Simply referencing a repo/branch is not good enough as HEAD changes over time.

Describe the solution you'd like
Being able to specify a git SHA to use.

Describe alternatives you've considered
Tags?

Additional context
There doesn't seem a reliable way of determining if a string is a SHA or a branch name as you can always name a branch using a SHA.

[chrisdenman]

Perhaps a tag is good enough here?