This mostly just adds a couple of debug traces and upgrades one debug() to info()
This should hopefully make it more obvious when a deployment repo is using git_rev
setting in scap.cfg to deploy something like origin/master instead of HEAD.
See T181661 where debugging the issue was made more difficult than necessary.
I failed to notice that the rORESDEPLOY scap/scap.cfg had git_rev = origin/master
but they did did not have git_upstream_submodules.
I've also added docs for git_rev to repo_config.rst and reorganized
the "available config variables" table to better group related vars.