Add a setting to squash Gerrit uploads by default, and a --no-squash command line option to override it.

BUG=nope

Review URL: https://codereview.chromium.org/1584703005

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@298260 0039d316-1c4b-4281-b951-d872f2087c98