Clone a branch or commit hash from config

[jjspark]

Allows cloning a branch or commit hash when configuring repos in config.yml

[xorpaul]

Thanks for the pull request.

So you want to specify a specific branch as the source git repository.
I presume the -branch parameter while executing g10k is not enough for this and you want to hard-code it in the g10k config?

Hard-coding a specific commit hash via ref: does make sense.