tests/revnamesext.py

revlog: add a C implementation of `headrevsdiff`

Python implementation of `headrevsdiff` can be very slow in the worst
case compared with the `heads` computation it replaces, since the
latter is done in C.

Even the average case of this Python implementation is still
noticeable in the profiles.

This patch makes the computation much much faster by doing it in C.

# Dummy extension to define a namespace containing revision names


from mercurial import namespaces


def reposetup(ui, repo):
    names = {b'r%d' % rev: repo[rev].node() for rev in repo}
    namemap = lambda r, name: names.get(name)
    nodemap = lambda r, node: [b'r%d' % repo[node].rev()]

    ns = namespaces.namespace(
        b'revnames',
        templatename=b'revname',
        logname=b'revname',
        listnames=lambda r: names.keys(),
        namemap=namemap,
        nodemap=nodemap,
    )
    repo.names.addnamespace(ns)