FR: Add a language for selecting a tree

[martinvonz]

Is your feature request related to a problem? Please describe.

It's always bothered me that jj diff -r X cannot be written as jj diff --to X --from <something> because X- might resolve to multiple revisions. There are other such examples like in jj restore.

A related to case that came up on Discord was that jj restore could be used for resolving a conflict in favor of a particular side of the conflict if we had a way of selecting a given tree from a conflict.

Describe the solution you'd like

Commands that currently take a revision argument only to resolve it to a tree (e.g. jj diff --from/--to, jj restore --from, jj file show -r) could take a new kind of expression instead of a revset. The new expression would allow regular revset but will resolve to a tree instead. By default, the revset is expected to resolve to a single revision and we use that revision's tree (merged if a conflict). But the user can also do something like jj diff --from 'merge_of(X-)' --to Y to merge the commits inX-, or jj restore --from 'conflict_side(X, 1)'.

Describe alternatives you've considered

We've talked about making revsets themselves able to calculate merges of commits. I don't like that idea because it complicates revsets, and it means that the merged result would presumably be assigned to a commit (with some placeholder description etc) to fit into the revset language.

[arxanas]

A related to case that came up on Discord was that jj restore could be used for resolving a conflict in favor of a particular side of the conflict if we had a way of selecting a given tree from a conflict.

Link to the Discord question thread: "Resolve conflict by picking a side for all".