Sketch of change to migrate `git-init` to script mode (https://github.com/tektoncd/pipeline/issues/1961)

git_resource.go

// GetInputTaskModifier returns the TaskModifier to be used when this resource is an input.
func (s *GitResource) GetInputTaskModifier(_ *TaskSpec, path string) (TaskModifier, error) {
        return &InternalTaskModifier{
                StepsToPrepend: []Step{{
                        Container: corev1.Container{
                                Name:       names.SimpleNameGenerator.RestrictLengthWithRandomSuffix(gitSource + "-" + s.Name),
                                Image:      s.GitImage,
                                WorkingDir: pipeline.WorkspaceDir,
                                // This is used to populate the ResourceResult status.
                                Env: []corev1.EnvVar{{
                                        Name:  "TEKTON_RESOURCE_NAME",
                                        Value: s.Name,
                                }, {
                                        Name:  "URL",
                                        Value: s.URL,
                                }, {
                                        Name:  "REVISION",
                                        Value: s.Revision,
                                }, {
                                        Name:  "PATH",
                                        Value: s.Path,
                                }},
                        },
                        Script: `#!/usr/bin/env bash
set -euxo pipefail

if [[ ${PATH} != "" ]]; do
  git init ${PATH}
  cd ${PATH}
else
  git init
done

git remote add origin ${URL}
# TODO: sslVerify
# TODO: submodules=false

git fetch --recurse-submodules=yes origin ${REVISION}
# TODO: depth

# TODO: if git fetch failed, do a full clone and git checkout

git reset --hard FETCH_HEAD

# Write git commit SHA to termination message path.
commit=$(git rev-parse HEAD)
echo "commit ${commit}" >> /tekton/termination
`,      
                }},
        }, nil

}