compare tag location with head of its branch

Instead of assuming the master branch, use the actual branch containing
the tag when looking for unreleased changes. This makes the list-changes
report more useful release candidates on a stable branch during the
period when the default series has not been changed (so releases on
stable/a show the right info when the default branch is not b).

Change-Id: I0b5857cce27cd021c59bafe26abfb471f1899311
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

openstack_releases/cmds/list_changes.py

@@ -307,13 +307,16 @@ def main():
                 )
                 print('HEAD of {} is {}'.format(tag_branch, head_sha))
             else:
+                if branch in branches:
+                    tag_branch = branch
+                else:
+                    tag_branch = branches[0]
                 head_sha = gitutils.sha_for_tag(
                     workdir,
                     project['repo'],
-                    'HEAD',
+                    tag_branch,
                 )
-                print('HEAD of {} is {}'.format(branch, head_sha))
+                print('HEAD of {} is {}'.format(tag_branch, head_sha))
-                tag_branch = branch
             requested_sha = gitutils.sha_for_tag(
                 workdir,
                 project['repo'],