revert: add a way for external extensions to prefetch file data

This allow extensions that mess with the storage layer (remotefilelog,
largefile) to prefetch any data that will be accessed during the revert
operation.

We are currently fetching more data than theoretically required because the
backup code is a bit stupid. Future patches will improve that.

mercurial/cmdutil.py

@@ -2527,6 +2527,9 b' def revert(ui, repo, ctx, parents, *pats'
             (unknown,    actions['unknown'],  discard),
             )
 
+        needdata = ('revert', 'add', 'remove', 'undelete')
+        _revertprefetch(repo, ctx, *[actions[name][0] for name in needdata])
+
         for abs, (rel, exact) in sorted(names.items()):
             # target file to be touch on disk (relative to cwd)
             target = repo.wjoin(abs)
@@ -2564,6 +2567,10 b' def revert(ui, repo, ctx, parents, *pats'
     finally:
         wlock.release()
 
+def _revertprefetch(repo, ctx, *files):
+    """Let extension changing the storage layer prefetch content"""
+    pass
+
 def _performrevert(repo, parents, ctx, actions):
     """function that actually perform all the actions computed for revert