This function doesn't actually need commits, it only needs their IDs. In some
contexts we may only have commit IDs, so there's no need to require an iterator
of Commits.
This commit also adds a `CommitIteratorExt` that makes it easy to convert an
iterator of `&Commit` to an iterator of `&CommitId`.
This change updates the language of `jj edit`'s help message to be
more clear as to the nature of the command. It also adds a
recommendation for a more idiomatic/safer workflow.
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.
It seems better to have the caller pass the transaction description
when we finish the transaction than when we start it. That way we have
all the information we want to include more readily available.
