convert: try to be smarter about CVS branching

Better handles this case:

The output from cvsps -A -u --cvs-direct -q:
---------------------
PatchSet 1
Date: 2008/02/08 20:33:28
Author: fk
Branch: HEAD
Tag: (none)
Log:
initial

Members:
         file_one:INITIAL->1.1

---------------------
PatchSet 2
Date: 2008/02/08 20:33:32
Author: fk
Branch: branch_name
Ancestor branch: HEAD
Tag: (none)
Log:
new file on branch

Members:
         file_two:1.1->1.1.2.1

hgext/convert/cvs.py

@@ -71,7 +71,7 b' class convert_cvs(converter_source):'
                     elif l.startswith("Ancestor branch"):
                         ancestor = l[17:-1]
                         # figure out the parent later
-                        self.parent[id] = None
+                        self.parent[id] = self.lastbranch[ancestor]
                     elif l.startswith("Author"):
                         author = self.recode(l[8:-1])
                     elif l.startswith("Tag:") or l.startswith("Tags:"):
@@ -101,13 +101,14 b' class convert_cvs(converter_source):'
                             p = []
                         if branch == "HEAD":
                             branch = ""
-                        if branch and p[0] == None:
+                        if branch:
                             latest = None
                             # the last changeset that contains a base
                             # file is our parent
                             for r in oldrevs:
-                                latest = max(filerevids[r], latest)
-                            p = [latest]
+                                latest = max(filerevids.get(r, None), latest)
+                            if latest:
+                                p = [latest]
 
                         # add current commit to set
                         c = commit(author=author, date=date, parents=p,