pq: Don't fail --commit on empty commits

So far we would fail empty commits with the confusing error

$ gbp pq export --commit
gbp:info: Generating patches from git (debian/sid..patch-queue/debian/sid)
gbp:error: Error running git commit:

2 files changed, 34 insertions, 4 deletions

diff --git a/gbp/scripts/pq.py b/gbp/scripts/pq.py
index 945cf1ed..37968edd 100755
--- a/gbp/scripts/pq.py
+++ b/gbp/scripts/pq.py
@@ -168,8 +168,10 @@ def commit_patches(repo, branch, patches, options, patch_dir):
     # FIXME: handle case were only the contents of the patches changed
     added, removed = compare_series(oldpatches, newpatches)
     msg = format_series_diff(added, removed, options)
-    repo.add_files(PATCH_DIR)
-    repo.commit_staged(msg=msg)
+
+    if not repo.is_clean(paths='debian/patches')[0]:
+        repo.add_files(PATCH_DIR)
+        repo.commit_staged(msg=msg)
     return added, removed
 
 
diff --git a/tests/13_test_gbp_pq.py b/tests/13_test_gbp_pq.py
index be87f909..c63ee12d 100644
--- a/tests/13_test_gbp_pq.py
+++ b/tests/13_test_gbp_pq.py
@@ -210,8 +210,12 @@ class TestWritePatch(testutils.DebianGitTestRepo):
 
 class TestExport(testutils.DebianGitTestRepo):
     class Options(object):
-        drop = True
+        drop = False
         patch_numbers = False
+        renumber = False
+        patch_num_format = ''
+        meta_closes = False
+        meta_closes_bugnum = ''
         pq_from = 'DEBIAN'
 
     def setUp(self):
@@ -223,12 +227,36 @@ class TestExport(testutils.DebianGitTestRepo):
         repo = self.repo
         start = repo.get_branch()
         pq_branch = os.path.join('patch-queue', start)
+        opts = TestExport.Options()
+        opts.drop = True
+
         pq.switch_pq(repo, start)
         self.assertEqual(repo.get_branch(), pq_branch)
-        export_patches(repo, pq_branch, TestExport.Options)
+        export_patches(repo, pq_branch, opts)
         self.assertEqual(repo.get_branch(), start)
         self.assertFalse(repo.has_branch(pq_branch))
 
+    def test_commit(self):
+        """Test if we commit the patch-queue branch with --commit"""
+        repo = self.repo
+        start = repo.get_branch()
+        pq_branch = os.path.join('patch-queue', start)
+        opts = TestExport.Options()
+        opts.commit = True
+        pq.switch_pq(repo, start)
+        self.assertEqual(len(repo.get_commits()), 1)
+        self.assertEqual(repo.get_branch(), pq_branch)
+        self.add_file('foo', 'foo')
+        export_patches(repo, pq_branch, opts)
+        self.assertEqual(repo.get_branch(), start)
+        self.assertTrue(repo.has_branch(pq_branch))
+        self.assertEqual(len(repo.get_commits()), 2,
+                         "Export did not create commit")
+        export_patches(repo, pq_branch, opts)
+        self.assertEqual(repo.get_branch(), start)
+        self.assertEqual(len(repo.get_commits()), 2,
+                         "Export must not create another commit")
+
 
 class TestParseGbpCommand(unittest.TestCase):
     def test_empty_body(self):