Mock GetChangeDetail instead of using self.calls to make the code
easier to change.

Bug: 1051631
Change-Id: I77361caccaf694644839825d890343864267688f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2062716
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Anthony Polito <apolito@google.com>