sidedata: add a `decode_files_sidedata` function

Right now the function mostly gather existing code to build a consistent object.
However having this function allow us prepare all user code independently from
the actual side data format change (and associated encoding/decoding changes)

Strictly speaking, we could not need to passe the sidedata explicitly since we
have access to it though the `changelogrevision` object. However, the short
term goal is to drop that first parameter to only pass the sidedata binaries.

mercurial/changelog.py

@@ -306,6 +306,9 b' class changelogrevision(object):'
     def changes(self):
         if self._changes is not None:
             return self._changes
+        if self._cpsd:
+            changes = metadata.decode_files_sidedata(self, self._sidedata)
+        else:
         changes = metadata.ChangingFiles(
             touched=self.files or (),
             added=self.filesadded or (),

mercurial/metadata.py

@@ -342,6 +342,32 b' def encode_files_sidedata(files):'
     return sidedata
 
 
+def decode_files_sidedata(changelogrevision, sidedata):
+    """Return a ChangingFiles instance from a changelogrevision using sidata
+    """
+    touched = changelogrevision.files
+
+    rawindices = sidedata.get(sidedatamod.SD_FILESADDED)
+    added = decodefileindices(touched, rawindices)
+
+    rawindices = sidedata.get(sidedatamod.SD_FILESREMOVED)
+    removed = decodefileindices(touched, rawindices)
+
+    rawcopies = sidedata.get(sidedatamod.SD_P1COPIES)
+    p1_copies = decodecopies(touched, rawcopies)
+
+    rawcopies = sidedata.get(sidedatamod.SD_P2COPIES)
+    p2_copies = decodecopies(touched, rawcopies)
+
+    return ChangingFiles(
+        touched=touched,
+        added=added,
+        removed=removed,
+        p1_copies=p1_copies,
+        p2_copies=p2_copies,
+    )
+
+
 def _getsidedata(srcrepo, rev):
     ctx = srcrepo[rev]
     filescopies = computechangesetcopies(ctx)