kythe tool: only show binding definitions, by default
kythe tool: only show binding definitions, by default

Summary: This behavior is less surprising than what appears to be duplicate definitions showing.

Reviewers: #core_team, fromberger

Reviewed By: #core_team, fromberger

Subscribers: #core_team

Differential Revision: https://phabricator-dot-kythe-repo.appspot.com/D1712

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