Set source in coveragerc rather than passing on command-line.

.coveragerc

@@ -1,5 +1,6 @@
 [run]
 branch = True
+source = dulwich
 
 [report]
 exclude_lines =

.travis.yml

@@ -27,11 +27,11 @@ install:
 
 script:
   # Test without c extensions
-  - python -m coverage run -p --source=dulwich -m unittest dulwich.tests.test_suite
+  - python -m coverage run -p -m unittest dulwich.tests.test_suite
 
   # Test with c extensions
   - python setup.py build_ext -i
-  - python -m coverage run -p --source=dulwich -m unittest dulwich.tests.test_suite
+  - python -m coverage run -p -m unittest dulwich.tests.test_suite
 
   # Style
   - make style

Makefile

@@ -65,7 +65,7 @@ before-push: check
 	git diff origin/master | $(PEP8) --diff
 
 coverage:
-	$(COVERAGE) run --source=dulwich -m unittest dulwich.tests.test_suite dulwich.contrib.test_suite
+	$(COVERAGE) run -m unittest dulwich.tests.test_suite dulwich.contrib.test_suite
 
 coverage-html: coverage
 	$(COVERAGE) html