This adds pluggable codereview-specific implementations into
Changelist class. The specific implementation is chosen at
Changelist automatically, with Rietveld being default for
backwards compatibility.

Gerrit implementation for Gerrit is incomplete, and will be
added in later CLs. However, it is sufficient to ensure
current functionality of this tool is not diminished.

Sadly, the base class isn't completely free from Rietveld
assumptions because of presubmit_support. Apparently, PRESUBMIT 
scripts can make use of Rietveld instance for RPCs directly.
This use doesn't make sense for Gerrit, which substitutes
rietveld instance with a dummy object, which raises exception
on any attribute access with a diagnostic message.

This also includes refactoring of some related code which
(ab)used ChangeList. Overall, this CL adds a few extra call to
git config in order to determine which codereview to use, but
but it shouldn't have any performance impact.


BUG=579160

Review URL: https://codereview.chromium.org/1805193002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@299462 0039d316-1c4b-4281-b951-d872f2087c98