[git] Fix switching branch of shallow clone (#18728)

* [git] Fix fetching branch of shallow clone

* Use absolute file:// paths to make sure git uses shallow clones

* Improve tests

* Fix sanity errors
* Match style according to other (depth) tests

* Improve tests

Now they will fail without the fix of this PR

test/integration/targets/git/tasks/depth.yml

@@ -171,3 +171,54 @@
   file:
     state: absent
     path: "{{ checkout_dir }}"
+
+#
+# Make sure shallow fetch works when switching to (fetching) a new a branch
+#
+
+- name: DEPTH | clone from branch with depth specified
+  git:
+    repo: 'file://{{ repo_dir|expanduser }}/shallow_branches'
+    dest: '{{ checkout_dir }}'
+    depth: 1
+    version: test_branch
+
+- name: DEPTH | check if clone is shallow
+  stat: path={{ checkout_dir }}/.git/shallow
+  register: is_shallow
+  when: git_version.stdout is version(git_version_supporting_depth, '>=')
+
+- name: DEPTH | assert that clone is shallow
+  assert:
+    that:
+      - is_shallow.stat.exists
+  when: git_version.stdout is version(git_version_supporting_depth, '>=')
+
+- name: DEPTH | switch to new branch (fetch) with the shallow clone
+  git:
+    repo: 'file://{{ repo_dir|expanduser }}/shallow_branches'
+    dest: '{{ checkout_dir }}'
+    depth: 1
+    version: new_branch
+  register: git_fetch
+
+- name: DEPTH | assert if switching a shallow clone to a new branch worked
+  assert:
+    that:
+      - git_fetch is changed
+
+- name: DEPTH | check if clone is still shallow
+  stat: path={{ checkout_dir }}/.git/shallow
+  register: is_shallow
+  when: git_version.stdout is version(git_version_supporting_depth, '>=')
+
+- name: DEPTH | assert that clone still is shallow
+  assert:
+    that:
+      - is_shallow.stat.exists
+  when: git_version.stdout is version(git_version_supporting_depth, '>=')
+
+- name: DEPTH | clear checkout_dir
+  file:
+    state: absent
+    path: "{{ checkout_dir }}"