When debugging behavior of badly-GCed repos, I find it's annoying that "op log"
fails because the index can't be loaded. Since "op log" doesn't need a repo, I
think it's better to display the exact op-heads state without merging.
I thought this would be done by dag_walk::topo_order_reverse_lazy_ok(), but
apparently I made it preserve the input order in a way topo_order_reverse()
would do.
The OpStore backends should have a better way to look up operation by id than
traversing from the op heads. The added method is similar to the commit Index
one, but returns an OpStoreResult because the backend operation can fail.
FWIW, if we want .shortest() in the op log template, we'll probably need a
trait method that returns an OpIndex instead.
I'm going to add try_from_hex(), which requires Self: Sized. Such trait bound
could be added, but I don't think we'll need abstracted ObjectId constructors
at all.
I'm going to add a prefix resolution method to OpStore, but OpStore is
unrelated to the index. I think ObjectId, HexPrefix, and PrefixResolution can
be extracted to this module.
This will be used in "jj op abandon ..op_id" command. The "op_id..@" range will
be reparented onto the root operation.
The current implementation is good enough for local repos, but it won't scale.
We might want to extract it as a trait method or introduce OpIndex for
efficient DAG operation.
A possible use case is when doing some archaeology around a certain operation.
The current implementation is quadratic if + is repeated. Suppose op_id is
usually close to the current op heads, I think it'll practically work better
than building a reverse lookup table.
This removes CommandError dependency from these resolution functions. We might
want to refactor the error types again if we introduce a real "opset" evaluator.
The error message for unresolved op heads now includes "@" instead of the whole
expression.
