hgweb: extract code for emitting multiple changelist records
changelistentry() exists so extensions can easily monkeypatch the
function to add additional metadata.
In at least one case at Mozilla, we have an extension doing this
where performance would greatly benefit if we were able to "batch
fetch" metadata for all revisions that will eventually have their
data set.
By extracting the logic for "obtain records for N revisions" into
a standalone function, we make it possible for a monkeypatched
function to more efficiently bulk fetch data for N revisions.
Differential Revision: https://phab.mercurial-scm.org/D3573
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