allow tracking branch names when the branches only exist on origin

Fix a problem with branch references so that it is now possible to use a
local tracking branch name when the branch only exists on the 'origin'
remote. For example, this allows references to 'stable/ocata' when there
is no local branch with that name but there is an 'origin/stable/ocata'
branch.

Change-Id: If11a0ff10ad3b1dfc82f99e6f68684ec9268af26
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

releasenotes/notes/allow-short-branch-names-61a35be55f04cea4.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fix a problem with branch references so that it is now possible to
+    use a local tracking branch name when the branch only exists on
+    the 'origin' remote. For example, this allows references to
+    'stable/ocata' when there is no local branch with that name but
+    there is an 'origin/stable/ocata' branch.

reno/scanner.py

@@ -487,6 +487,10 @@ class Scanner(object):
                 'refs/tags/' + name,
                 # If a stable branch was removed, look for its EOL tag.
                 'refs/tags/' + (name.rpartition('/')[-1] + '-eol'),
+                # If someone is using the "short" name for a branch
+                # without a local tracking branch, look to see if the
+                # name exists on the 'origin' remote.
+                'refs/remotes/origin/' + name,
             ]
             for ref in candidates:
                 key = ref.encode('utf-8')

reno/tests/test_scanner.py

@@ -1416,6 +1416,42 @@ class BranchTest(Base):
         self.assertIsNotNone(head2)
         self.assertEqual(head1, head2)
 
+    def test_remote_branch_without_prefix(self):
+        self.repo.git('checkout', '2.0.0')
+        self.repo.git('checkout', '-b', 'stable/2')
+        self.repo.git('checkout', 'master')
+        scanner1 = scanner.Scanner(self.c)
+        head1 = scanner1._get_ref('stable/2')
+        self.assertIsNotNone(head1)
+        print('head1', head1)
+        # Create a second repository by cloning the first.
+        print(utils.check_output(
+            ['git', 'clone', self.reporoot, 'reporoot2'],
+            cwd=self.temp_dir,
+        ))
+        reporoot2 = os.path.join(self.temp_dir, 'reporoot2')
+        print(utils.check_output(
+            ['git', 'remote', 'update'],
+            cwd=reporoot2,
+        ))
+        print(utils.check_output(
+            ['git', 'remote', '-v'],
+            cwd=reporoot2,
+        ))
+        print(utils.check_output(
+            ['find', '.git/refs'],
+            cwd=reporoot2,
+        ))
+        print(utils.check_output(
+            ['git', 'branch', '-a'],
+            cwd=reporoot2,
+        ))
+        c2 = config.Config(reporoot2)
+        scanner2 = scanner.Scanner(c2)
+        head2 = scanner2._get_ref('stable/2')
+        self.assertIsNotNone(head2)
+        self.assertEqual(head1, head2)
+
 
 class ScanStopPointPrereleaseVersionsTest(Base):