Introduce a getdep command to get revision/version/variable value information
as a counterpart for gclient setdep.

It will be useful for autorollers.

Bug: 760633
Change-Id: Iabeae0e78c6fbdcb1a3a79cfb380ac2d83f256d5
Reviewed-on: https://chromium-review.googlesource.com/999171Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>