Tracking followups from comments on merge+diff here. Can instead break this into separate issues for each one if preferred.

Short-term followups:

• Update progress for merge and diff. Draft PR for merge here.
• Update inline cache for merge and diff. Draft PR here.
• Documentation updates, with format as described here.
• Unlazy merge+diff refs right away if they will be needed for a solve. Comment here.
• Support returning direct nil from solver op execs. Comment here.

Performance followups:

• Deduplicate equivalent merges
• Deduplicate equivalent diffs
• Re-use submerges when possible, i.e. if Merge(a,b) already exists, use it to speed up creation of Merge(a,b,c)
• Normalize merges to their simplest form, i.e. Merge(a,b,a) is logically equivalent to the simpler Merge(b,a)
• Don't finalize input refs when they are created by llb.Local. Comment here.

Longer-term followups:

• Cache diff metadata and use it to apply diffs instead of always running the double-walking or overlay differ every time. Comment here
• Deduplicate code between diffApply and the containerd/continuity appliers+differs. Comment here.