remotefilelog: log when we're about to fetch files

I'm debugging a slow client situation and knowing how many files are
in the batch request would be a nice thing.

Differential Revision: https://phab.mercurial-scm.org/D6353

hgext/remotefilelog/fileserverclient.py

@@ -396,6 +396,9 b' class fileserverclient(object):'
                                 batchdefault = 10
                             batchsize = self.ui.configint(
                                 'remotefilelog', 'batchsize', batchdefault)
+                            self.ui.debug(
+                                b'requesting %d files from '
+                                b'remotefilelog server...\n' % len(missed))
                             _getfilesbatch(
                                 remote, self.receivemissing, progress.increment,
                                 missed, idmap, batchsize)