changegroup: add functionality to skip adding changelog data to changegroup
In narrow extension, when we have a non-ellipses narrow working copy and we
extend it, we pull all the changelog data again and the client tries to reapply
all that changelog data.
While downloading millions of changeset data is still not very expensive but
applying them on the client side is very expensive and takes ~10 minutes. These
10 minutes are added to every `hg tracked --addinclude <>` call and extending
a narrow copy becomes very slow.
This patch adds a new changelog argument to cgpacker.generate() fn. If the
changelog argument is set to False, we won't yield the changelog data. We still
have to iterate over the deltas returned by _generatechangelog() because that's
a generator and builds the data for clstate variable which is required for
calculating manifests and filelogs.
Differential Revision: https://phab.mercurial-scm.org/D4638
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