Adding `new_from_merged` constructor to InputDigests (#15549)
Adding `new_from_merged` constructor to InputDigests (#15549)

This is some groundwork for the "Batched" process future change.

The uncached one-process-per-file requests will have their input digests merged to form the input digests of one mega-process.
benjyw/pants,pantsbuild/pants,pantsbuild/pants,benjyw/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,pantsbuild/pants,benjyw/pants,benjyw/pants,pantsbuild/pants,benjyw/pants,pantsbuild/pants,benjyw/pants