base-commit when prerequisite-patch-id is specified

[Konstantin Ryabitsev <konstantin@linuxfoundation.org>]

Hello:

I'm working on automatically supporting dependencies with b4, and I'm curious
if "base-commit" should be used to indicate where to start applying
dependencies, or if that's purely for the patch series itself.

E.g.:

    base-commit: [commit-ish]
    prerequisite-patch-id: abcd...1234
    prerequisite-patch-id: cdef...3456

In this situation, does base-commit mean:
1. where to apply abcd...1234
2. the sha we should find when we apply both prerequisite patches

In theory, #2 makes no sense, because we don't know where to apply
prerequisite-patch-id's and therefore can't reasonably arrive at whatever is
specified as "base-commit".

I expect the answer is "it's whatever you think it should be," because
prerequisite-patch-id's aren't really widely used, but I'll be happy to learn
that there's an established practice.

-K