This enables significant time savings, especially since file_list only exists to
enable file-specific hooks (which, AFAIK, nothing actually uses). On a z620
(linux) using the cached git repos, a first-time `gclient sync --nohooks` takes:
  * (with)    131.06s user 14.10s  system 117% cpu 2:03.89 total
  * (without) 482.13s user 189.35s system 144% cpu 7:45.63 total

This change makes nohooks cause file_list to be None if we don't need to
accumulate it, and updates GitWrapper and SvnWrapper appropriately.

R=szager@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/18541006

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@210026 0039d316-1c4b-4281-b951-d872f2087c98