Revert of Stop prepending "r" to commit hashes in merge_to_branch.py (patchset...

Revert of Stop prepending "r" to commit hashes in merge_to_branch.py (patchset #1 id:1 of https://codereview.chromium.org/1298973007/ )

Reason for revert:
Result when using the script:

tools/release/merge_to_branch.py
  File "tools/release/merge_to_branch.py", line 108
    self["revision_list"] = ", ".join(self["full_revision_list"]))
                                                                 ^
SyntaxError: invalid syntax

Original issue's description:
> Stop prepending "r" to commit hashes in merge_to_branch.py
>
> R=machenbach@chromium.org
>
> Committed: https://crrev.com/e5dbfd06c3dab5934e846c40af2a8b7fb5cdf0d6
> Cr-Commit-Position: refs/heads/master@{#30483}

TBR=machenbach@chromium.org,adamk@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/1321343002

Cr-Commit-Position: refs/heads/master@{#30505}

tools/release/merge_to_branch.py

@@ -104,8 +104,9 @@ class CreateCommitMessage(Step):
 
   def RunStep(self):
 
-    # Stringify: ["abcde", "12345"] -> "abcde, 12345"
-    self["revision_list"] = ", ".join(self["full_revision_list"]))
+    # Stringify: [123, 234] -> "r123, r234"
+    self["revision_list"] = ", ".join(map(lambda s: "r%s" % s,
+                                      self["full_revision_list"]))
 
     if not self["revision_list"]:  # pragma: no cover
       self.Die("Revision list is empty.")