Switch to range from xrange for python 3 compatibility

xrange no longer exists in python 3, and since it is a small number of
iterations there is a limited performance impact under python 2 by
using range instead.

Add check to ensure linearisation exception does not occur under python
3, due to exception in attempting to use a non existing function.

Change-Id: I3e6165b8bce14a44459fc9419792784f64636f5e

git_upstream/lib/importupstream.py

@@ -237,7 +237,7 @@ class ImportUpstream(LogDedentMixin, GitMixin):
             ancestors.add(root)
 
             # look for merge commits that are not part of ancestry path
-            for idx in xrange(counter - 1, -1, -1):
+            for idx in range(counter - 1, -1, -1):
                 commit = sequence[idx]
                 # if there is only one parent, no need to check the others
                 if len(commit.parents) < 2:

git_upstream/tests/commands/import/test_import.py

@@ -67,6 +67,10 @@ class TestImportCommand(TestWithScenarios, BaseTestCase):
         self.assertThat(self.logger.output,
                         Contains("Starting execution of import command"))
 
+        self.assertThat(
+            self.logger.output,
+            Not(Contains("Exception occurred during linearisation")))
+
         # perform sanity checks on results
         self._check_tree_state()