[collection] Allow a relative path as git remote

[pocke]

rbs collection didn't allow a relative path as a git remote. For example:

# rbs_collection.yaml
sources:
  - name: ruby/gem_rbs_collection
    remote: ../../path/to/gem_rbs_collection/
    revision: main
    repo_dir: gems

I need to specify relative paths for test of ruby/gems_rbs_collection repo. In this repository, I'd like to refer to the repository itself for the test.

So this patch makes a relative path available as git remote.