api: Add scm module with all_{X} methods for listing revs.

tests/func/api/test_scm.py

@@ -0,0 +1,30 @@
+from dvc.api.scm import all_branches, all_commits, all_tags
+
+
+def test_all_branches(tmp_dir, scm, dvc):
+    assert all_branches() == ["master"]
+
+    with tmp_dir.branch("branch", new=True):
+        tmp_dir.scm_gen("branch", "branch", "commit")
+
+    assert all_branches() == ["branch", "master"]
+
+
+def test_all_commits(tmp_dir, scm, dvc):
+    first = scm.get_rev()
+    assert all_commits() == [first]
+
+    tmp_dir.scm_gen("foo", "foo", "commit")
+    second = scm.get_rev()
+
+    assert set(all_commits()) == {first, second}
+
+
+def test_all_tags(tmp_dir, scm, dvc):
+    scm.tag("v1")
+    assert all_tags() == ["v1"]
+
+    tmp_dir.scm_gen("foo", "foo", "commit")
+    scm.tag("v2")
+
+    assert set(all_tags()) == {"v1", "v2"}