Fix test : do not consider deleted migrations

* Change way to detect deleted migrations (diff-filter of 'git diff' command)

tests/test_project_fixtures/test_deleted_migration_project/git/COMMIT_EDITMSG

@@ -1 +1,16 @@
 Delete file and insert new
+
+# Please enter the commit message for your changes. Lines starting
+# with '#' will be ignored, and an empty message aborts the commit.
+#
+# Date:      Thu Jan 18 23:27:37 2018 +0100
+#
+# On branch master
+# Changes to be committed:
+#	deleted:    test_app/migrations/0002_new_not_null_field.py
+#	new file:   test_app/migrations/0003_new_nullable_field.py
+#	modified:   test_app/models.py
+#
+# Untracked files:
+#	git/
+#

tests/test_project_fixtures/test_deleted_migration_project/git/logs/HEAD

@@ -1,3 +1,4 @@
 0000000000000000000000000000000000000000 0395cbee60f2ea1b97f42ebdd43991c8f5d8d76e David-Wobrock <david.wobrock@gmail.com> 1516314144 +0100	commit (initial): Project init
 0395cbee60f2ea1b97f42ebdd43991c8f5d8d76e 154ecf6119325cc3b1f3f5a4e709bfbd61a4a4ba David-Wobrock <david.wobrock@gmail.com> 1516314322 +0100	commit: Add backward incompatible migration
 154ecf6119325cc3b1f3f5a4e709bfbd61a4a4ba be2fae887ee7ecd2d1e6371af1a865c17414bde9 David-Wobrock <david.wobrock@gmail.com> 1516314457 +0100	commit: Delete file and insert new
+be2fae887ee7ecd2d1e6371af1a865c17414bde9 4bdc4ba9e7695893423be1b54f3101d6ccbd11c7 David-Wobrock <david.wobrock@gmail.com> 1516459290 +0100	commit (amend): Delete file and insert new

tests/test_project_fixtures/test_deleted_migration_project/git/logs/refs/heads/master

@@ -1,3 +1,4 @@
 0000000000000000000000000000000000000000 0395cbee60f2ea1b97f42ebdd43991c8f5d8d76e David-Wobrock <david.wobrock@gmail.com> 1516314144 +0100	commit (initial): Project init
 0395cbee60f2ea1b97f42ebdd43991c8f5d8d76e 154ecf6119325cc3b1f3f5a4e709bfbd61a4a4ba David-Wobrock <david.wobrock@gmail.com> 1516314322 +0100	commit: Add backward incompatible migration
 154ecf6119325cc3b1f3f5a4e709bfbd61a4a4ba be2fae887ee7ecd2d1e6371af1a865c17414bde9 David-Wobrock <david.wobrock@gmail.com> 1516314457 +0100	commit: Delete file and insert new
+be2fae887ee7ecd2d1e6371af1a865c17414bde9 4bdc4ba9e7695893423be1b54f3101d6ccbd11c7 David-Wobrock <david.wobrock@gmail.com> 1516459290 +0100	commit (amend): Delete file and insert new

tests/test_project_fixtures/test_deleted_migration_project/git/objects/4b/330bb1c7154afb23f170218d170dae079c437f

@@ -0,0 +1,2 @@
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tests/test_project_fixtures/test_deleted_migration_project/git/refs/heads/master

@@ -1 +1 @@
-be2fae887ee7ecd2d1e6371af1a865c17414bde9
+4bdc4ba9e7695893423be1b54f3101d6ccbd11c7