transaction: introduce "changes" dictionary to precisely track updates

The transaction is already tracking some data intended for hooks (in
'hookargs'). However, that information is minimal as we optimise for
passing data to other processes through environment variables. There are
multiple places were we could use more complete and lower level
information locally (eg: cache update, better report of changes to
hooks, etc...).

For this purpose we introduces a 'changes' dictionary on the
transaction.  It is intended to track every changes happening to the
repository (eg: new revs, bookmarks move, phases move, obs-markers,
etc).

For now we just adds the 'changes' dictionary. We'll adds more tracking
and usages over time.

mercurial/transaction.py

@@ -137,6 +137,10 b' class transaction(object):'
             releasefn = lambda tr, success: None
         self.releasefn = releasefn
 
+        # A dict dedicated to precisely tracking the changes introduced in the
+        # transaction.
+        self.changes = {}
+
         # a dict of arguments to be passed to hooks
         self.hookargs = {}
         self.file = opener.open(self.journal, "w")