[presubmit] Use GerritAccessor in CheckChangedLUCIConfigs

For tryjobs, determine the remote branch using GerritAccessor

Bug: 761488
Change-Id: Ieea6028d222af0014ec3a57c3ad7b7eb8af5ec33
Reviewed-on: https://chromium-review.googlesource.com/834995
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

presubmit_canned_checks.py

@@ -1244,13 +1244,16 @@ def CheckChangedLUCIConfigs(input_api, output_api):
   LUCI_CONFIG_HOST_NAME = 'luci-config.appspot.com'
 
   cl = git_cl.Changelist()
-  remote, remote_branch = cl.GetRemoteBranch()
-  if remote_branch.startswith('refs/remotes/%s/' % remote):
-    remote_branch = remote_branch.replace(
-        'refs/remotes/%s/' % remote, 'refs/heads/', 1)
-  if remote_branch.startswith('refs/remotes/branch-heads/'):
-    remote_branch = remote_branch.replace(
-        'refs/remotes/branch-heads/', 'refs/branch-heads/', 1)
+  if input_api.change.issue and input_api.gerrit:
+    remote_branch = input_api.gerrit.GetDestRef(input_api.change.issue)
+  else:
+    remote, remote_branch = cl.GetRemoteBranch()
+    if remote_branch.startswith('refs/remotes/%s/' % remote):
+      remote_branch = remote_branch.replace(
+          'refs/remotes/%s/' % remote, 'refs/heads/', 1)
+    if remote_branch.startswith('refs/remotes/branch-heads/'):
+      remote_branch = remote_branch.replace(
+          'refs/remotes/branch-heads/', 'refs/branch-heads/', 1)
 
   remote_host_url = cl.GetRemoteUrl()
   if not remote_host_url:

presubmit_support.py

@@ -245,6 +245,14 @@ class GerritAccessor(object):
 
     return rev_info['commit']['message']
 
+  def GetDestRef(self, issue):
+    ref = self.GetChangeInfo(issue)['branch']
+    if not ref.startswith('refs/'):
+      # NOTE: it is possible to create 'refs/x' branch,
+      # aka 'refs/heads/refs/x'. However, this is ill-advised.
+      ref = 'refs/heads/%s' % ref
+    return ref
+
   def GetChangeOwner(self, issue):
     return self.GetChangeInfo(issue)['owner']['email']