Add --changes flag to buildbucket.py's put subcommand
Add --changes flag to buildbucket.py's put subcommand

This flag allows the user to specify a file which contains a JSON list of dicts to set for the "changes" property when scheduling the build.

BUG=493885

Review URL: https://codereview.chromium.org/1199963004

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@295788 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

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