Build using a specific git commit from gentoo tree

[Doug Freed]

Allow the submitter to pick a specific git "commitish" (git's terminology for anything that resolves to a commit ID, eg a commit ID itself, a branch name, a tag, etc) to use for the gentoo tree when building the requested package. For simplicity and security, this will be restricted to repo/gentoo and the github mirror (which allows PRs to be tested as well).