When running `git rev-parse REV^{commit}` in Windows, `^` must be
escaped.
However, it was escaped more times than necessary.

Split IsValidRevision to call ResolveCommit, add tests to ResolveCommit,
and run scm.py tests on Windows.

Change-Id: I761a820394c8b5410d68b6ccd6c352c41c30c88c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2092904Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>