blob: Convert LFS pointer search by revision to use pseudo-revs in args

Historically, we couldn't pass pseudo-revisions ("--not", "--all") as normal positional arguments when searching for LFS pointers via git-rev-list(1) because their leading dash is disallowed by our git DSL. This MR creates the ability to override validation of positional arguments per git command and uses it to lift this restriction for git-rev-list(1).

While it is nice that we can get a nice code simplification for the LFS pointer RPCs, the overarching goal is that I want to introduce a new RPC call FindLFSPointers(revs []string), which is capable of replacing all three of the existing GetLFSPointers RPCs. This requires callers to mix and match pseudo-revisions with normal revisions though to be able to express what he previously did, which now becomes possible with this RPC.