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`.
When the caller needs a formatter, it's because they're doing
something non-trivial. When the user passed `--quiet` (see upcoming
patch), we should ideally skip doing related work for print the
formatting output. It helps if the `Ui` object doesn't even return a
`Formatter` then, so the caller is forced to handle the quiet case
differently.
Thanks to Yuya for the suggestion.
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.
