This changes adds the first pass of code needed for OWNERS files.
In total there should probably be maybe four user-visible changes:
  * new gcl/git-cl "suggest-reviewers" command
  * a presubmit hook on upload to automatically add the reviewers
  * an addition to gcl/git-cl status command that tells you
    which files still need review/approval.
  * a presubmit hook on commit to ensure all of the needed reviewers
    have approved the file.

This change implements a core "owners Database" object with the
dumbest possible algorithm for determining a covering set of reviewers,
and the skeleton of the presubmit hooks. This code will not be
used by anything yet, and is also missing unit tests.

Review URL: http://codereview.chromium.org/6581030

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@76342 0039d316-1c4b-4281-b951-d872f2087c98