fn through(text: &str, at: Rev) -> Result<&str, PrefixFailure>Expand description
The leading run of text’s lines that reaches no further than at.
A contiguous prefix, because that is what keeps the hash chain whole: a
line dropped from the middle would leave its successor naming a parent
that is no longer there. Which means the tags that survive are the ones
written before the cut — and a tag can only ever name a rev the history
already held, so every one of them names a rev at or before at. The
names given later to revs inside the cut are named_through’s to put
back.