remote-bzr: add custom method to find branches

The official method is incredibly inefficient and slow.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Felipe Contreras

contrib/remote-helpers/git-remote-bzr

@@ -756,6 +756,25 @@ def get_remote_branch(origin, remote_branch, name):
 
     return branch
 
+def find_branches(repo):
+    transport = repo.user_transport
+
+    for fn in transport.iter_files_recursive():
+        if not fn.endswith('.bzr/branch-format'):
+            continue
+
+        name = subdir = fn[:-len('/.bzr/branch-format')]
+        name = name if name != '' else 'master'
+        name = name.replace('/', '+')
+
+        try:
+            cur = transport.clone(subdir)
+            branch = bzrlib.branch.Branch.open_from_transport(cur)
+        except bzrlib.errors.NotBranchError:
+            continue
+        else:
+            yield name, branch
+
 def get_repo(url, alias):
     global dirname, peer, branches
 
@@ -796,11 +815,7 @@ def get_repo(url, alias):
     else:
         # repository
 
-        for branch in repo.find_branches():
-
-            name = repo.user_transport.relpath(branch.base)
-            name = name if name != '' else 'master'
-            name = name.replace('/', '+')
+        for name, branch in find_branches(repo):
 
             if not is_local:
                 peers[name] = branch