localrepo: introduce method for explicit branch cache update

Currently, localrepo.branchtags() is called in two locations to update the
_branchcache dict, however branchtags() itself does not update anything, it
calls branchmap() to do so. This change introduces a new updatebranchcache()
method that is used by both branchmap() and the calls to update the cache.

mercurial/localrepo.py

@@ -337,8 +337,7 b' class localrepository(repo.repository):'
 
         return partial
 
-    def branchmap(self):
-        '''returns a dictionary {branch: [branchheads]}'''
+    def updatebranchcache(self):
         tip = self.changelog.tip()
         if self._branchcache is not None and self._branchcachetip == tip:
             return self._branchcache
@@ -355,6 +354,9 b' class localrepository(repo.repository):'
         # this private cache holds all heads (not just tips)
         self._branchcache = partial
 
+    def branchmap(self):
+        '''returns a dictionary {branch: [branchheads]}'''
+        self.updatebranchcache()
         return self._branchcache
 
     def branchtags(self):
@@ -976,7 +978,7 b' class localrepository(repo.repository):'
             tr.close()
 
             if self._branchcache:
-                self.branchtags()
+                self.updatebranchcache()
             return n
         finally:
             if tr:
@@ -1700,7 +1702,7 b' class localrepository(repo.repository):'
         if changesets > 0:
             # forcefully update the on-disk branch cache
             self.ui.debug("updating the branch cache\n")
-            self.branchtags()
+            self.updatebranchcache()
             self.hook("changegroup", node=hex(cl.node(clstart)),
                       source=srctype, url=url)