[Enhancement] Allow specifying rev/tag/branch for templates

[matthme]

The degit crate used to fetch the template when using the --templates-url argument does not allow to specify rev/tag/branch which is quite a bummer when a template is supposed to support multiple holochain versions.

[c12i]

I think it probably has to do with degit not supporting this, and the project seems to not have been updated in three years.

To support specifying the /rev/tag/branch, we could probably migrate to using the git2 crate and cloning the repo using the RepoBuilder, then manually degit the cloned repo by deleting the .git directory.