sidedata: rename `encode_copies_sidedata` to `encode_files_sidedata`

We are storing more than copies information, so lets make it clear.

mercurial/changelog.py

@@ -570,7 +570,7 b' class changelog(revlog.revlog):'
         sortedfiles = sorted(files.touched)
         sidedata = None
         if self._copiesstorage == b'changeset-sidedata':
-            sidedata = metadata.encode_copies_sidedata(files)
+            sidedata = metadata.encode_files_sidedata(files)
 
         if extra:
             extra = encodeextra(extra)

mercurial/metadata.py

@@ -318,7 +318,7 b' def decodefileindices(files, data):'
         return None
 
 
-def encode_copies_sidedata(files):
+def encode_files_sidedata(files):
     sortedfiles = sorted(files.touched)
     sidedata = {}
     p1copies = files.copied_from_p1