update of corresponding github branch

[hufanglq]

update_all is very convenience. But when we have different branch of a github package in different location, update_all can only update the packages in .libPaths, and the installed other branch (ie: develop) will be updated to master branch. So maybe update_all can add option for lib.loc, the install_github of update_github add option for lib.loc and ref to access desc$GithubRef.