Simplify and improve the recipe for git-based builds

Modify the git-based recipe to simply mirror the remote git
repository.  If enabled by configuration, download and install the
newest official manpages and/or HTML docs as opposed to not installing
any documentation for git-based builds.

To save disk space and to simplify git operations, the repository
format of the local cached otp repository has been changed to bare
format.  Caller should execute "kerl cleanup all" if they have any
previously used the git-based build feature.