On Windows, git can be either an executable or batch script, each of
which requires carets (^) to be escaped in different ways.

This makes it hard to use `git rev-parse --verify REV^{commit}`, so we
truncate full hash revisions to achieve the same effect instead.

Change-Id: I01578620706a1bab75a2decc55d9b4f1ac3afe28
Bug: 1065307
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2127968Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>