filter-repo: work around git-fast-export bug

Explicitly specify --topo-order; git-fast-export fails on some topologies
unless it traverses in topological order.

Signed-off-by: Elijah Newren <newren@gmail.com>

git-filter-repo

@@ -322,7 +322,7 @@ class FastExportFilter(object):
         raise SystemExit("Could not parse line: '%s'" % self.nextline)
 
 def FastExportOutput(source_repo, extra_args = []):
-  return Popen(["git", "fast-export", "--all"] + extra_args,
+  return Popen(["git", "fast-export", "--all", "--topo-order"] + extra_args,
                stdout = PIPE,
                cwd = source_repo).stdout