This allows a script to generate a sufficient list of owners (and 
reviewers) for a given CL. Unlike the owners presubmit check and 
the normal, interactive mode of `git-cl owners`, we do not print 
the comments associated with an owner for each file, since 
presumably a script doesn't care about them (and the comments
aren't in a particularly machine-parsable form, anyway).

This could be used by the WPT import scripts, and presumably
by roll scripts and other such things.

I didn't add a test for this because there appears to be no good
mechanism for testing top-level CMD functions in git_cl that 
didn't end up just repeating the code mock-for-mock :(.

R: phajdan.jr@chromium.org
Change-Id: I3a9e6d117d6f4bafe938b07b028f119835c3e575
Reviewed-on: https://chromium-review.googlesource.com/648653Reviewed-by: Paweł Hajdan Jr. <phajdan.jr@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>