localrepo: add ignoremissing parameter to branchtip

Previously, in the namespaces api, the only caller of branchtip was singlenode
which happened to raise the same exception that branchtip raised: KeyError.

This is a minor change but will allow upcoming patches to use repo.branchtip to
not raise an exception if a branch doesn't exist. After that, it will be
possible for extensions to use the namespace api in a stable way.

mercurial/localrepo.py

@@ -718,12 +718,21 b' class localrepository(object):'
         branchmap.updatecache(self)
         return self._branchcaches[self.filtername]
 
-    def branchtip(self, branch):
-        '''return the tip node for a given branch'''
+    def branchtip(self, branch, ignoremissing=False):
+        '''return the tip node for a given branch
+
+        If ignoremissing is True, then this method will not raise an error.
+        This is helpful for callers that only expect None for a missing branch
+        (e.g. namespace).
+
+        '''
         try:
             return self.branchmap().branchtip(branch)
         except KeyError:
+            if not ignoremissing:
             raise error.RepoLookupError(_("unknown branch '%s'") % branch)
+            else:
+                pass
 
     def lookup(self, key):
         return self[key].node()

mercurial/namespaces.py

@@ -41,7 +41,7 b' class namespaces(object):'
 
         n = ns("branches", "branch",
                lambda repo: repo.branchmap().keys(),
-               lambda repo, name: tolist(repo.branchtip(name)),
+               lambda repo, name: tolist(repo.branchtip(name, True)),
                lambda repo, node: [repo[node].branch()])
         self.addnamespace(n)