The Create Change API currently supports the ability to specify
a `base_change`, and the change which you create will use that
existing change as its parent. However, it has no support for
specifying a commit sha1 which already exists on the target
branch.
This change introduces a new `base_commit` field, which takes
a 40 character sha1 as its value. It also introduces processing
to validate that the given sha1 is valid and exists on the
target branch. In also includes documentation and tests for the
above behavior.
Bug: Issue 8857
Change-Id: I7f1daa008b9a8d144c745327495dac65f581a5bf
e8e7328f38