support local mirrors, e.g. `git clone --reference`

[khilman]

Is your feature request related to a problem? Please describe.

A worker may have a local git mirror of large repositories. It would be useful for pipelines to be able to use worker-local git mirrors to dramatically speed up git clone operations. Reading the current docs, it seems there is currently no way to tell a git clone to use a local mirror (e.g. git clone --reference /path/to/mirror.git).

Describe the solution you'd like

Provide a way for a git resource to use a local mirror with a new property, such as mirror-path e.g.:

  - name: kernel-repo                                                                                                                                                                            
    type: git                                                                                                                                                                                    
    tags: ["worker-with-mirror"]                                                                                                                                                                            
    source:                                                                                                                                                                                      
      uri: ((kernel-repo-url))                                                                                                                                                                   
      branch: ((kernel-repo-branch))                                                                                                                                                             
      mirror-path: /path/to/local/mirror.git

Describe alternatives you've considered

I also considered just an way to pass arbitrary arguments to git but this seems like it might open up too many ways to shoot oneself in the foot.

Additional context

No response

[marco-m]

An alternative that should be doable with the git resource as-is is to have a git mirror, but not colocated with the worker(s), just in the same network of the workers, and point the uri to the mirror.

For example, say the original repo is at https://github.com/foo/bar and the mirror is at http://mirror.internal/foo/bar. Then you could just point the resource to the mirror...