Fix typo in merge script.

BUG=chromium:451975
TBR=tandrii@chromium.org
NOTRY=true
LOG=n

TEST=./script_test.py

Review URL: https://codereview.chromium.org/932703002

Cr-Commit-Position: refs/heads/master@{#26676}

tools/release/common_includes.py

@@ -339,10 +339,10 @@ class GitInterface(VCInterface):
     if name in ["candidates", "master"]:
       return "refs/remotes/origin/%s" % name
     # Check if branch is in heads.
-    if self.Git("show-ref refs/remotes/origin/%s" % name).strip():
+    if self.step.Git("show-ref refs/remotes/origin/%s" % name).strip():
       return "refs/remotes/origin/%s" % name
     # Check if branch is in branch-heads.
-    if self.Git("show-ref refs/remotes/branch-heads/%s" % name).strip():
+    if self.step.Git("show-ref refs/remotes/branch-heads/%s" % name).strip():
       return "refs/remotes/branch-heads/%s" % name
     self.Die("Can't find remote of %s" % name)