cbuildbot: name internal manifest branch attribute sanely.

The previous tracking_branch usage is overloaded and makes following
the code harder than it needs be.  The usage of this attribute is
literally for tracking what the target manifest branch is, thus
use that naming.

BUG=None
TEST=run_tests.sh
TEST=cbuildbot x86-generic-paladin --buildbot <usual-args>

Change-Id: Ib3dc1017878c37656a9d186a3999130289e463fa
Reviewed-on: https://gerrit.chromium.org/gerrit/21967
Tested-by: Brian Harring <ferringb@chromium.org>
Reviewed-by: Ryan Cui <rcui@chromium.org>
Commit-Ready: Brian Harring <ferringb@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index bfc16fc..3a0f286 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -185,7 +185,7 @@
     self.archive_stages = {}
     self.archive_urls = {}
     self.release_tag = None
-    self.tracking_branch = _GetChromiteTrackingBranch()
+    self.target_manifest_branch = _GetChromiteTrackingBranch()
     self.gerrit_patches = None
     self.local_patches = None
 
@@ -201,11 +201,11 @@
       self.gerrit_patches, self.local_patches = _PreProcessPatches(
           self.options.gerrit_patches, self.options.local_patches)
 
-    bs.BuilderStage.SetTrackingBranch(self.tracking_branch)
+    bs.BuilderStage.SetManifestBranch(self.target_manifest_branch)
 
     # Check branch matching early.
     if _IsIncrementalBuild(self.options.buildroot, self.options.clobber):
-      _CheckBuildRootBranch(self.options.buildroot, self.tracking_branch)
+      _CheckBuildRootBranch(self.options.buildroot, self.target_manifest_branch)
 
     self._RunStage(stages.CleanUpStage)