updaterepo: add deprecation warning

We just moved all in-tree callers off of the function, but we're
leaving it here for a bit to help extension writers.

Differential Revision: https://phab.mercurial-scm.org/D9110

mercurial/hg.py

@@ -1049,6 +1049,10 b' def updaterepo(repo, node, overwrite, up'
     When overwrite is set, changes are clobbered, merged else
 
     returns stats (see pydoc mercurial.merge.applyupdates)"""
+    repo.ui.deprecwarn(
+        b'prefer merge.update() or merge.clean_update() over hg.updaterepo()',
+        b'5.7',
+    )
     return mergemod._update(
         repo,
         node,