This CL makes a couple changes:
  * The goal here is to be able to specify git cache entirely from the
    environment variable $GIT_CACHE_PATH and not require special treatment from
    bot_update. Eventually this will be specified at the swarming task level
    instead of in the recipe (i.e. "cached git" will eventually be an
    implementation detail of git on the bots and completely transparent to
    all other software).
  * Removal of the general --cache-dir option from gclient. This option was
    error-prone; it doesn't actually make sense to configure this on
    a per-invocation basis. The sole exception was `gclient config`, which
    now allows this option to be set directly.
  * Consolidation of GitWrapper.cache_dir and GitWrapper._GetMirror; previously
    these two things could disagree, leading to weird intermediate states. Now
    they're the same value.

R=agable@chromium.org, ehmaldonado@chromium.org, tandrii@chromium.org

Bug: 823434
Change-Id: I14adc7619b5fc10768ce32be2651c6215ba94aff
Reviewed-on: https://chromium-review.googlesource.com/1105473Reviewed-by: Aaron Gable <agable@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>