Skip the _CheckPolicyIDs[1] check if the CL is a merge CL.

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Rationale:
_CheckPolicyIDs[1] comment says:
Holes would not be a technical problem, but we want to ensure that nobody accidentally omits IDs.

It only makes sense to ensure this on master. When merging back, it's possible to leave out policies that are not being merged back.

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How:
Check the remote branch, skip the check if it is not origin/master.

[1] https://cs.chromium.org/chromium/src/components/policy/tools/syntax_check_policy_template_json.py?rcl=554991dec6dc951153addfa064f4c7f18cc9cbac&l=149