context: schedule file prefetch before comparing for cleanliness When using a system like remotefilelog, we can occasionally run into scenarios where the local content cache does not have the data we need to perform these comparisons. These will be fetched on-demand, but individually; if the remotefilelog server isn't extremely low latency, the runtime of the command becomes dominated by the multiple getfile requests for individual files. By performing the prefetch, we can download these files in bulk, and save server resources and many network roundtrips. Differential Revision: https://phab.mercurial-scm.org/D5532