Allow to skip revision check in roll-dep.
Allow to skip revision check in roll-dep.

Without this change, roll-dep requires dependencies to be
checked out and up to date in order to roll a newer revision.

This change allows to use roll-dep with less checking and
makes the script user responsible to verify the correctness
of the deps path and git hash.

This eases writing a deps roller that updates multiple deps
including a variety of os deps, which might not be checked
out on the host of the deps roller.

BUG=438705, 437462
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/801643004

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@293444 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools