Ignore submodules when diffing

By default, git diff includes entries for submodules in its output.
This breaks when trying to extract information from such a file
since the submodule directory is not in fact a file known to git.
This may be related to a GitPython issue:
gitpython-developers/GitPython#891

But for git-hammer, the sensible course of action seems to be
ignoring submodules. They are not code in the main repository,
and if their code should be in a project, the actual repository
can be included. So add to diff the option ignore-submodules that
causes it to skip submodules.

Also add a unit test that creates a submodule in the test
repository and verifies that git-hammer works correctly on it.

Author: Jaakko Kangasharju

githammer/hammer.py

@@ -146,7 +146,7 @@ def _make_full_commit_stats(self, repository, commit):
 
     def _make_diffed_commit_stats(self, repository, commit, previous_commit, previous_commit_line_counts,
                                   previous_commit_test_counts):
-        diff_index = previous_commit.diff(commit, w=True)
+        diff_index = previous_commit.diff(commit, w=True, ignore_submodules=True)
         current_files = set()
         previous_files = set()
         for add_diff in diff_index.iter_change_type('A'):
@@ -202,7 +202,7 @@ def _add_commit_object(self, repository, commit, session):
         if len(commit.parents) <= 1:
             if len(commit.parents) == 1:
                 diff_stat = repository.git_repository.git.diff(
-                    commit.parents[0], commit, numstat=True)
+                    commit.parents[0], commit, numstat=True, ignore_submodules=True)
             else:
                 diff_stat = repository.git_repository.git.show(commit, numstat=True, format='')
             added_lines = 0

tests/__init__.py

@@ -1,2 +1,3 @@
 from .test_init import HammerInitTest
 from .test_single_repository import HammerRepositoryTest
+from .test_submodule import HammerSubmoduleTest

tests/data/.gitignore

@@ -1,3 +1,3 @@
-# When working on the test repository, clone it to the directory working-tree
-# here, so git will ignore it.
-working-tree
+# When working on the test repository, clone it to a directory starting with
+# worktree here, so git will ignore it.
+worktree*

tests/data/subrepository/HEAD

@@ -0,0 +1 @@
+ref: refs/heads/master

tests/data/subrepository/config

@@ -0,0 +1,6 @@
+[core]
+	repositoryformatversion = 0
+	filemode = true
+	bare = true
+	ignorecase = true
+	precomposeunicode = true

tests/data/subrepository/description

@@ -0,0 +1 @@
+Unnamed repository; edit this file 'description' to name the repository.

tests/data/subrepository/info/exclude

@@ -0,0 +1,6 @@
+# git ls-files --others --exclude-from=.git/info/exclude
+# Lines that start with '#' are comments.
+# For a project mostly in C, the following would be a good set of
+# exclude patterns (uncomment them if you want to use them):
+# *.[oa]
+# *~

tests/data/subrepository/objects/30/3804d461da9cdfef86f6053d0ad2d0545adae1

@@ -0,0 +1,3 @@
+x
=��
+�0�a�y�ً�I��B��N}�I:��HL�Ƿ �:�/��ò	��]��`��ulO����c��)��'UtdZS)+(�>D���
+�k�C�k���DUjT���,�[ݞG�MS��Oz���8��t9�8��`���E{�� n���G�/yt:

tests/data/subrepository/objects/7b/2efdbe6118db73a3be33b641ecca24fa6d6927

@@ -0,0 +1 @@
+303804d461da9cdfef86f6053d0ad2d0545adae1

tests/data/subrepository/objects/81/8c2582af6d5e5f7f3ab139c9fe1331fa1b577f

@@ -0,0 +1,21 @@
+import os
+import git
+
+from .hammer_test import HammerTest
+
+
+class HammerSubmoduleTest(HammerTest):
+
+    def setUp(self):
+        super().setUp()
+        git.Repo.clone_from(os.path.join(self.current_directory, 'data', 'repository'),
+                            os.path.join(self.working_directory.name, 'worktree'))
+        self.repository = git.Repo(os.path.join(self.working_directory.name, 'worktree'))
+        git.Submodule.add(self.repository, 'subrepo', 'subrepo',
+                          os.path.join(self.current_directory, 'data', 'subrepository'))
+        author = git.Actor('Author A', '[email protected]')
+        self.repository.index.commit('Add subrepo', author=author)
+
+    def test_repository_with_submodule_is_understood(self):
+        self.hammer.add_repository(os.path.join(self.working_directory.name, 'worktree'))
+        self.assertIsNotNone(self.hammer.head_commit())