llvm/utils: guarantee revert_checker's revert ordering

At the moment, the revert ordering from this tool is unspecified (though
it happens to be in `git log` order, so newest reverts come first).

From the standpoint of tooling and users, this seems to be the opposite
of what we want by default: tools and users will generally try to apply
these reverts as cherry-picks. If two reverts in the list are close
enough to each other, if the reverts get applied out of order, we'll get
a merge conflict.

Rather than having `reverse`s for all tools (and mental reverses for
manual users), just guarantee an oldest-first output ordering for this
function.

Differential Revision: https://reviews.llvm.org/D106838