Allow setting arbitrary 'git cl try' properties with buildbucket.
Allow setting arbitrary 'git cl try' properties with buildbucket.

This allows calling 'git cl try' with properties. Each
property is set with the option -p key=value, where value
will be treated as json if possible or as string.

Examples for -p and pitfalls:

key=string -> 'string'
key=1 -> 1
key=[1,2,3] -> [1,2,3]
key=["one","two","three"] -> '[one,two,three]'
'key=["one","two","three"]' -> ['one','two','three']

BUG=488235

Review URL: https://codereview.chromium.org/1339073002

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@296683 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

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