remotefilelog: limit number of changesets to be prefetched
Added config option 'prefetchdays' which indicates that commits older than
'prefetchdays' days should not be prefetched. This option is necessary to avoid
prefetch of huge amount of data. The default value is set to 14 days.
Test Plan: Ensure that unit tests pass
Reviewers: ryanmce, simonfar, durham, #fbhgext, simpkins
Reviewed By: #fbhgext, simpkins
Differential Revision: https://phab.mercurial-scm.org/D420