Adapt to status becoming an object in Mercurial 5.3

Status has always been a tuple, but since 5.3, commit:
https://www.mercurial-scm.org/repo/hg/rev/c5548b0b6847, it is an object.
Therefore the __getitem__ of the tuple isn't available anymore.

This fix is compatible with mercurial>=4.6, as the old status tuple
still has the same properties.

hg-fast-export.py

@@ -320,8 +320,8 @@ def get_branchname(name):
       # later non-merge revision: feed in changed manifest
       # if we have exactly one parent, just take the changes from the
       # manifest without expensively comparing checksums
-      f=repo.status(parents[0],revnode)[:3]
-      added,changed,removed=f[1],f[0],f[2]
+      f=repo.status(parents[0],revnode)
+      added,changed,removed=f.added,f.modified,f.removed
       type='simple delta'
     else: # a merge with two parents
       wr('merge %s' % revnum_to_revref(parents[1], old_marks))