Remove force push from GitCheckout.commit()
Remove force push from GitCheckout.commit()

The CQ should not be able to force push -- this allows it to wipe out legitimate
commits if something goes wrong internally. This happened recently, with a CQ
commit orphaning 20 commits from the previous few days. This may have happened
any number of times previously, probably only wiping out one commit at a time,
whenever there was a race between the CQ and a developer's direct commit+push.

Repositories should also protect themselves by disallowing force push in their
ACLs.

BUG=261619

Review URL: https://codereview.chromium.org/196573041

git-svn-id: fd409f4bdeea2bb50a5d34bb4d4bfc2046a5a3dd@258092 0039d316-1c4b-4281-b951-d872f2087c98

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