Make bin/phd debug quieter by default

Summary:
By default, bin/phd debug activates --trace, which is incredibly verbose.

Instead, be moderately verbose by default, and only include tracing if --trace was passed to bin/phd debug.

See also D15422.

Test Plan:

• Ran bin/phd debug task, got moderate amount of most useful debug output.
• Ran bin/phd debug task --trace, got very verbose, detailed low-level debug output.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15423