cmdutil: convert _revertprefetch() to a generic stored file hook (API)

This will be used by LFS to fetch required files in a group for multiple
commands, prior to being accessed.  That avoids the one-at-a-time fetch when the
filelog wrapper goes to access it, and it is missing locally (which costs two
round trips to the server.)  The core command list that needs this is probably
at least:

  - annotate
  - archive (which is also used by extdiff)
  - cat
  - diff
  - export
  - grep
  - verify (sadly)
  - anything that has the '{data}' template

There are no core users of the revert prefetch hook, and never have been since
it was introduced in 45e02cfad4bd for remotefilelog.  Thanks to Yuya for
figuring out a way to reliably trigger the deprecated warning.  Unfortunately,
it wanted to blame the caller of revert.  Passing along an adjusted stack level
seemed the least bad choice (although it still blames a core function).

One thing to note is that the store lock isn't being held when this is called.
I'm not at all familiar with remotefilelog or its locking requirements, so this
may not be a big deal.  Currently, LFS doesn't hold a lock when downloading
files.  Even though largefiles doesn't either, I'm starting to think it should,
and maybe the .hg/store/lock isn't good enough to cover the globally shared
cache.

.. api::

   The cmdutil._revertprefetch() hook point for prefetching stored files has
   been replaced by the command agnostic cmdutil._prefetchfiles().  The new
   function takes a list of files, instead of a list of lists of files.

mercurial/cmdutil.py

@@ -2862,7 +2862,14 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])
+            if _revertprefetch is not _revertprefetchstub:
+                ui.deprecwarn("'cmdutil._revertprefetch' is deprecated, use "
+                              "'cmdutil._prefetchfiles'", '4.6', stacklevel=1)
+                _revertprefetch(repo, ctx,
+                                *[actions[name][0] for name in needdata])
+            oplist = [actions[name][0] for name in needdata]
+            _prefetchfiles(repo, ctx,
+                           [f for sublist in oplist for f in sublist])
             _performrevert(repo, parents, ctx, actions, interactive, tobackup)
 
         if targetsubs:
@@ -2875,8 +2882,15 b' def revert(ui, repo, ctx, parents, *pats'
                     raise error.Abort("subrepository '%s' does not exist in %s!"
                                       % (sub, short(ctx.node())))
 
-def _revertprefetch(repo, ctx, *files):
-    """Let extension changing the storage layer prefetch content"""
+def _revertprefetchstub(repo, ctx, *files):
+    """Stub method for detecting extension wrapping of _revertprefetch(), to
+    issue a deprecation warning."""
+
+_revertprefetch = _revertprefetchstub
+
+def _prefetchfiles(repo, ctx, files):
+    """Let extensions changing the storage layer prefetch content for any non
+    merge based command."""
 
 def _performrevert(repo, parents, ctx, actions, interactive=False,
                    tobackup=None):

mercurial/ui.py

@@ -1614,7 +1614,7 b' class ui(object):'
                      msg, *calframe[stacklevel][1:4])
             curframe = calframe = None  # avoid cycles
 
-    def deprecwarn(self, msg, version):
+    def deprecwarn(self, msg, version, stacklevel=2):
         """issue a deprecation warning
 
         - msg: message explaining what is deprecated and how to upgrade,
@@ -1625,7 +1625,7 b' class ui(object):'
             return
         msg += ("\n(compatibility will be dropped after Mercurial-%s,"
                 " update your code.)") % version
-        self.develwarn(msg, stacklevel=2, config='deprec-warn')
+        self.develwarn(msg, stacklevel=stacklevel, config='deprec-warn')
 
     def exportableenviron(self):
         """The environment variables that are safe to export, e.g. through