repo: add get_subrepo_relpath helper for studio live experiments monorepo support

tests/unit/repo/test_repo.py

@@ -135,3 +135,51 @@ def test_dynamic_cache_initialization(tmp_dir, scm):
     dvc.close()
 
     Repo(str(tmp_dir)).close()
+
+
+def test_monorepo_relpath(tmp_dir, scm):
+    from dvc.repo.destroy import destroy
+
+    tmp_dir.gen({"project_a": {}, "subdir/project_b": {}})
+
+    non_monorepo = Repo.init(tmp_dir)
+    assert non_monorepo.subrepo_relpath == ""
+
+    destroy(non_monorepo)
+
+    monorepo_project_a = Repo.init(tmp_dir / "project_a", subdir=True)
+
+    assert monorepo_project_a.subrepo_relpath == "project_a"
+
+    monorepo_project_b = Repo.init(tmp_dir / "subdir" / "project_b", subdir=True)
+
+    assert monorepo_project_b.subrepo_relpath == "subdir/project_b"
+
+
+def test_virtual_monorepo_relpath(tmp_dir, scm):
+    from dvc.fs.git import GitFileSystem
+    from dvc.repo.destroy import destroy
+
+    tmp_dir.gen({"project_a": {}, "subdir/project_b": {}})
+    scm.commit("initial commit")
+    gfs = GitFileSystem(scm=scm, rev="master")
+
+    non_monorepo = Repo.init(tmp_dir)
+    non_monorepo.fs = gfs
+    non_monorepo.root_dir = "/"
+
+    assert non_monorepo.subrepo_relpath == ""
+
+    destroy(non_monorepo)
+
+    monorepo_project_a = Repo.init(tmp_dir / "project_a", subdir=True)
+    monorepo_project_a.fs = gfs
+    monorepo_project_a.root_dir = "/project_a"
+
+    assert monorepo_project_a.subrepo_relpath == "project_a"
+
+    monorepo_project_b = Repo.init(tmp_dir / "subdir" / "project_b", subdir=True)
+    monorepo_project_b.fs = gfs
+    monorepo_project_b.root_dir = "/subdir/project_b"
+
+    assert monorepo_project_b.subrepo_relpath == "subdir/project_b"