Consider a jit-diff mode where disassembly is done in the respective base/diff repo

As from https://github.com/dotnet/jitutils/issues/216, it might be useful to expose a mode where base/diff is done in the respective repos rather than by copying the base jit into the diff repo test environment.

This would help with analyzing purely managed code changes and cases where the jit interface has changed (which prevents the jits from being interchanged).