cubicweb-vcsfile #3852390 refresh-repo needs update for recent evolve extension [deprecated]

the evolve extension is growing some smarts to avoid transferring all obsolete markers at each pull/push. this is going to clash with the way vcsfile does a two-step pull to avoid getting changesets without the corresponding markers, since the second pull (with -r null) will no longer pull any markers. So that second pull will need to be replaced with something like "hg pull -r <my-heads>".

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