Implicitly concatenated strings are not supported when setting new
revisions, so raise an error when attempting to do se.

Bug: 1060772
Change-Id: I74d39b9eb8dcc420e9b7787c90bee642beb783cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2099407Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>