inject_extensions should use true user ID for commits

[thomasgoodfellow]

The script was written to fetch the user name and email from the git config, however in gitlab builds this failed since these run as root (which is a separate source of pain). Rather than configure git for root the quick hack was switching to the bogus user "bob". This should instead be a command option, so that GitLab builds can specify it and default use attributes it correctly to the user, e.g. equivalent override for build could be:

--git-user=Bob --git-email=[email protected]

although something more descriptive would be better:

--git-user=gitlab-runner --git-email=[email protected]