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

mercurial/context.py

@@ -2018,6 +2018,12 b' class overlayworkingctx(committablectx):'
         to resolve a conflict.
         """
         keys = []
+        # This won't be perfect, but can help performance significantly when
+        # using things like remotefilelog.
+        scmutil.prefetchfiles(
+            self.repo(), [self.p1().rev()],
+            matchmod.match('', '', patterns=self._cache.keys(), exact=True))
+
         for path in self._cache.keys():
             cache = self._cache[path]
             try: