Add prepare-workspace-git role

Currently the incremental push of the git repos to the nodes is split
into a part in project-config and a part in zuul-jobs and only works
within the OpenStack context. The prepare-workspace-git role is an
attempt to make this generically available in zuul-jobs.

It first checks for each needed project if it's cached. If yes it's
cloned to the destination, if not it's just initialized as an empty
git repo. After that we can generically use git operations to sync the
correct state of the repos to the destination.

Change-Id: I87938a02d51e561b25ca2cb20f53d62f3cd3ae73

roles/prepare-workspace-git/README.rst

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+Mirror the local git repos to remote nodes
+
+This role uses git operations (unlike :zuul:role:`prepare-workspace`
+which uses rsync) to mirror the locally prepared git repos to the remote
+nodes while taking advantage of cached repos on the node if they exist.
+This role works generically regardless of the existence of a cached
+repo on the node.
+
+The cached repos need to be placed using the canonical name under the
+`cached_repos_root` directory.
+
+**Role Variables**
+
+.. zuul:rolevar:: cached_repos_root
+   :default: /opt/git
+
+   The root of the cached repos.

roles/prepare-workspace-git/defaults/main.yaml

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+cached_repos_root: /opt/git

roles/prepare-workspace-git/tasks/main.yaml

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+- name: Find locally cached git repos
+  stat:
+    path: "{{ cached_repos_root }}/{{ item.canonical_name }}"
+  with_items: "{{ zuul.projects.values() | list }}"
+  register: cached_repos
+
+# We do a bare clone here first so that we skip creating a working copy that
+# will be overwritten later anyway.
+- name: Clone cached repo to workspace
+  shell: |
+    set -e
+    git clone --bare {{ cached_repos_root }}/{{ item.0.canonical_name }} {{ ansible_user_dir }}/{{ item.0.src_dir}}/.git
+    cd {{ ansible_user_dir }}/{{ item.0.src_dir }}
+    git config --local --bool core.bare false
+  args:
+    creates: "{{ ansible_user_dir }}/{{ item.0.src_dir}}"
+  when: item.1.stat.exists
+  with_together:
+    - "{{ zuul.projects.values() | list }}"
+    - "{{ cached_repos.results }}"
+  # ANSIBLE0006: If we use the git module, we get warning
+  # ANSIBLE0004 since we  do not give an explicit version
+  tags:
+    - skip_ansible_lint
+
+- name: Initialize non-cached repos
+  command: "git init {{ ansible_user_dir }}/{{ item.0.src_dir}}"
+  args:
+    creates: "{{ ansible_user_dir }}/{{ item.0.src_dir}}"
+  when: not item.1.stat.exists
+  with_together:
+    - "{{ zuul.projects.values() | list }}"
+    - "{{ cached_repos.results }}"
+  # ANSIBLE0006: If we use the git module, we get warning
+  # ANSIBLE0004 since we  do not give an explicit version
+  tags:
+    - skip_ansible_lint
+
+- name: Remove origin from local git repos and replace it by the zuul fake origin
+  # To be idempotent, remove origin only if it's found in the local list.
+  shell: |
+    set -e
+    git remote -v | grep origin && git remote rm origin || true
+    git remote add origin file:///dev/null
+  args:
+    chdir: "{{ ansible_user_dir }}/{{ item.src_dir}}"
+  with_items: "{{ zuul.projects.values() | list }}"
+  # ANSIBLE0006: git remote is not supported by ansible module
+  tags:
+    - skip_ansible_lint
+
+# TODO(tobiash): we might want to deprecate the role mirror-workspace-git-repos
+# and move it here.
+- name: Synchronize repos
+  import_role:
+    name: mirror-workspace-git-repos