repoview: extract actual hidden cache writing in its own function

This will allow the generation of this cache within the transaction. Relying on
the transaction will reduce the chance of reader seeing bad cache.

mercurial/repoview.py

@@ -72,6 +72,13 b' def cachehash(repo, hideable):'
     h.update(str(hash(frozenset(hideable))))
     return h.digest()
 
+def _writehiddencache(cachefile, cachehash, hidden):
+    """write hidden data to a cache file"""
+    data = struct.pack('>%ii' % len(hidden), *sorted(hidden))
+    cachefile.write(struct.pack(">H", cacheversion))
+    cachefile.write(cachehash)
+    cachefile.write(data)
+
 def trywritehiddencache(repo, hideable, hidden):
     """write cache of hidden changesets to disk
 
@@ -87,12 +94,8 b' def trywritehiddencache(repo, hideable, '
             wlock = repo.wlock(wait=False)
             # write cache to file
             newhash = cachehash(repo, hideable)
-            sortedset = sorted(hidden)
-            data = struct.pack('>%ii' % len(sortedset), *sortedset)
             fh = repo.vfs.open(cachefile, 'w+b', atomictemp=True)
-            fh.write(struct.pack(">H", cacheversion))
-            fh.write(newhash)
-            fh.write(data)
+            _writehiddencache(fh, newhash, hidden)
         except (IOError, OSError):
             repo.ui.debug('error writing hidden changesets cache')
         except error.LockHeld: