[MERGE] fast-path the common case of _synchronise_history

[Robert Collins]

_synchronize_history is called with a revision_id that is not-None in
all cases I could find; this means that the fast path of using
last_revision_info is never triggered; this patch enables a check of
last_revision_info as a cheap way to map the common case.

-Rob

=== modified file 'bzrlib/branch.py'
--- bzrlib/branch.py    2007-08-20 13:07:12 +0000
+++ bzrlib/branch.py    2007-08-22 07:50:19 +0000
@@ -2034,14 +2034,18 @@
             # we know the revno without needing to walk all of history
             revno = source_revno
         else:
-            # To figure out the revno for a random revision, we need to build
-            # the revision history, and count its length.
-            # We don't care about the order, just how long it is.
-            # Alternatively, we could start at the current location, and count
-            # backwards. But there is no guarantee that we will find it since
-            # it may be a merged revision.
-            revno = len(list(self.repository.iter_reverse_revision_history(
-                                                                revision_id)))
+            # usually when revision_id is supplied it is the tip - try 
+            # for the fast_path
+            revno, tip_revision_id = self.last_revision_info()
+            if tip_revision_id != revision_id:
+                # To figure out the revno for a random revision, we need to
+                # build the revision history, and count its length.  We don't
+                # care about the order, just how long it is.  Alternatively, we
+                # could start at the current location, and count backwards. But
+                # there is no guarantee that we will find it since it may be a
+                # merged revision.
+                revno = len(list(self.repository.iter_reverse_revision_history(
+                                                                    revision_id)))
         destination.set_last_revision_info(revno, revision_id)
 
     def _make_tags(self):


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