cmdutil: convert the prefetchfiles() hook to a callback mechanism (API)

Yuya suggested a list of callbacks instead of function wrapping.  This means
that one extension can't block another from processing the list.

.. api::

   File prefetching is now handled by registering a callback with
   scmutil.fileprefetchhooks.

mercurial/cmdutil.py

@@ -2865,8 +2865,9 b' def revert(ui, repo, ctx, parents, *pats'
         if not opts.get('dry_run'):
             needdata = ('revert', 'add', 'undelete')
             if _revertprefetch is not _revertprefetchstub:
-                ui.deprecwarn("'cmdutil._revertprefetch' is deprecated, use "
-                              "'cmdutil._prefetchfiles'", '4.6', stacklevel=1)
+                ui.deprecwarn("'cmdutil._revertprefetch' is deprecated, "
+                              "add a callback to 'scmutil.fileprefetchhooks'",
+                              '4.6', stacklevel=1)
                 _revertprefetch(repo, ctx,
                                 *[actions[name][0] for name in needdata])
             oplist = [actions[name][0] for name in needdata]
@@ -2893,6 +2894,7 b' def _revertprefetchstub(repo, ctx, *file'
 def _prefetchfiles(repo, ctx, files):
     """Let extensions changing the storage layer prefetch content for any non
     merge based command."""
+    scmutil.fileprefetchhooks(repo, ctx, files)
 
 def _performrevert(repo, parents, ctx, actions, interactive=False,
                    tobackup=None):

mercurial/scmutil.py

@@ -1222,6 +1222,11 b' class simplekeyvaluefile(object):'
     'unbundle',
 ]
 
+# a list of (repo, ctx, files) functions called by various commands to allow
+# extensions to ensure the corresponding files are available locally, before the
+# command uses them.
+fileprefetchhooks = util.hooks()
+
 # A marker that tells the evolve extension to suppress its own reporting
 _reportstroubledchangesets = True