repos: cleannup and fix landing-ref code.

with latest git update this exposed a bug when we allowed to set references like tip for git repos
now the defaults are loaded from a single place controlled by default branch for git.

Repos without master are now working just fine with new git and new logic.

rhodecode/api/tests/test_update_repo.py

@@ -22,6 +22,7 @@ import mock
 import pytest
 
 from rhodecode.model.repo import RepoModel
+from rhodecode.model.scm import ScmModel
 from rhodecode.tests import TEST_USER_ADMIN_LOGIN, TEST_USER_REGULAR_LOGIN
 from rhodecode.api.tests.utils import (
     build_data, api_call, assert_error, assert_ok, crash, jsonify)
@@ -65,8 +66,8 @@ class TestApiUpdateRepo(object):
         ({'push_uri': 'http://example.com/repo_push'},
          {'push_uri': 'http://example.com/repo_push'}),
 
-        ({'landing_rev': 'rev:tip'},
-         {'landing_rev': ['rev', 'tip']}),
+        ({'landing_rev': None},  # auto-updated based on type of repo
+         {'landing_rev': [None, None]}),
 
         ({'enable_statistics': True},
          SAME_AS_UPDATES),
@@ -97,6 +98,12 @@ class TestApiUpdateRepo(object):
         if updates.get('_group'):
             fixture.create_repo_group(updates['_group'])
 
+        if 'landing_rev' in updates:
+            default_landing_ref, _lbl = ScmModel.backend_landing_ref(backend.alias)
+            _type, _name = default_landing_ref.split(':')
+            updates['landing_rev'] = default_landing_ref
+            expected['landing_rev'] = [_type, _name]
+
         expected_api_data = repo.get_api_data(include_secrets=True)
         if expected is SAME_AS_UPDATES:
             expected_api_data.update(updates)