revert: move prefetch to after the actions logic

The prefetch logic came before the actual population of the actions collection,
so it was always being passed an empty action list. This fixes it by moving it
to after that logic.

The only consumer of this function at the moment is remotefilelog, and I
verified it works with this change.

mercurial/cmdutil.py

@@ -2765,9 +2765,6 b' def revert(ui, repo, ctx, parents, *pats'
             (unknown,       actions['unknown'],  discard),
             )
 
-        needdata = ('revert', 'add', 'undelete')
-        _revertprefetch(repo, ctx, *[actions[name][0] for name in needdata])
-
         wctx = repo[None]
         for abs, (rel, exact) in sorted(names.items()):
             # target file to be touch on disk (relative to cwd)
@@ -2797,6 +2794,9 b' def revert(ui, repo, ctx, parents, *pats'
 
 
         if not opts.get('dry_run'):
+            needdata = ('revert', 'add', 'undelete')
+            _revertprefetch(repo, ctx, *[actions[name][0] for name in needdata])
+
             _performrevert(repo, parents, ctx, actions)
 
             # get the list of subrepos that must be reverted