branchmap: make update responsible to update the cache key

The update function have all necessary data to keep the branchcache key
up to date with its value.

This saves assignment to the cache key that each caller of update had to do by
hand.

The strip case is a bit more complicated to handles from inside the function but
I do not expect any impact.

mercurial/branchmap.py

@@ -91,17 +91,39 b' def update(repo, partial, ctxgen):'
             if ancestors:
                 bheadrevs = [b for b in bheadrevs if b not in ancestors]
         partial[branch] = [cl.node(rev) for rev in bheadrevs]
+        tiprev = max(bheadrevs)
+        if tiprev > partial.tiprev:
+            partial.tipnode = cl.node(tiprev)
+            partial.tiprev = tiprev
+
 
     # There may be branches that cease to exist when the last commit in the
     # branch was stripped.  This code filters them out.  Note that the
     # branch that ceased to exist may not be in newbranches because
     # newbranches is the set of candidate heads, which when you strip the
     # last commit in a branch will be the parent branch.
+    droppednodes = []
     for branch in partial.keys():
         nodes = [head for head in partial[branch]
                  if cl.hasnode(head)]
         if not nodes:
+            droppednodes.extend(nodes)
             del partial[branch]
+    try:
+        node = cl.node(partial.tiprev)
+    except IndexError:
+        node = None
+    if ((partial.tipnode != node)
+        or (partial.tipnode in droppednodes)):
+        # cache key are not valid anymore
+        partial.tipnode = nullid
+        partial.tiprev = nullrev
+        for heads in partial.values():
+            tiprev = max(cl.rev(node) for node in heads)
+            if tiprev > partial.tiprev:
+                partial.tipnode = cl.node(tiprev)
+                partial.tiprev = tiprev
+
 
 def updatecache(repo):
     repo = repo.unfiltered()  # Until we get a smarter cache management
@@ -121,8 +143,6 b' def updatecache(repo):'
     if partial.tiprev < catip:
         ctxgen = (repo[r] for r in cl.revs(partial.tiprev + 1, catip))
         update(repo, partial, ctxgen)
-        partial.tipnode = cl.node(catip)
-        partial.tiprev = catip
         partial.write(repo)
     # If cacheable tip were lower than actual tip, we need to update the
     # cache up to tip. This update (from cacheable to actual tip) is not
@@ -131,8 +151,6 b' def updatecache(repo):'
     if partial.tiprev < tiprev:
         ctxgen = (repo[r] for r in cl.revs(partial.tiprev + 1, tiprev))
         update(repo, partial, ctxgen)
-        partial.tipnode = cl.node(tiprev)
-        partial.tiprev = tiprev
     repo._branchcache = partial
 
 class branchcache(dict):

mercurial/localrepo.py

@@ -1438,8 +1438,6 b' class localrepository(object):'
                       if self.changelog.hasnode(node))
             cache = self._branchcache
             branchmap.update(self, cache, ctxgen)
-            cache.tipnode = self.changelog.tip()
-            cache.tiprev = self.changelog.rev(cache.tipnode)
             cache.write(self)
 
         # Ensure the persistent tag cache is updated.  Doing it now