always convert the chunks cache we find

it seems it is possible that the chunks files are copied but *not*
converted. this may have happened here because the conversion was
interrupted, although the specific scenario is still unclear (but it
did happen during manual tests here). therefore reproducing this
problem seems to be difficult, hence the lack of tests for this
specific issue.

since we consider the header replacement code to be safe, that we
always convert shouldn't pose any additional threat to the existing
borg chunk cache.

borg/upgrader.py

@@ -166,23 +166,23 @@ class AtticRepositoryUpgrader(Repository):
             :params path: the basename of the cache file to copy
             (example: "files" or "chunks") as a string
 
-            :returns: the borg file that was created or None if non
-            was created.
+            :returns: the borg file that was created or None if no
+            Attic cache file was found.
 
             """
             attic_file = os.path.join(attic_cache_dir, path)
             if os.path.exists(attic_file):
                 borg_file = os.path.join(borg_cache_dir, path)
                 if os.path.exists(borg_file):
-                    logger.warning("borg cache file already exists in %s, skipping conversion of %s" % (borg_file, attic_file))
+                    logger.warning("borg cache file already exists in %s, not copying from Attic" % (borg_file))
                 else:
                     logger.info("copying attic cache file from %s to %s" % (attic_file, borg_file))
                     if not dryrun:
                         shutil.copyfile(attic_file, borg_file)
-                    return borg_file
+                return borg_file
             else:
                 logger.warning("no %s cache file found in %s" % (path, attic_file))
-            return None
+                return None
 
         # XXX: untested, because generating cache files is a PITA, see
         # Archiver.do_create() for proof
@@ -196,11 +196,10 @@ class AtticRepositoryUpgrader(Repository):
 
             # we need to convert the headers of those files, copy first
             for cache in ['chunks']:
-                copied = copy_cache_file(cache)
-                if copied:
-                    logger.info("converting cache %s" % cache)
-                    if not dryrun:
-                        AtticRepositoryUpgrader.header_replace(cache, b'ATTICIDX', b'BORG_IDX')
+                cache = copy_cache_file(cache)
+                logger.info("converting cache %s" % cache)
+                if not dryrun:
+                    AtticRepositoryUpgrader.header_replace(cache, b'ATTICIDX', b'BORG_IDX')
 
 
 class AtticKeyfileKey(KeyfileKey):