Add tests for parse_unified_diff

We could probably use non-git diff fixtures, but those are what we have
for now.

tests/fixtures/diffs/single_file_diff

@@ -0,0 +1,27 @@
+diff --git a/flake8/utils.py b/flake8/utils.py
+index f6ce384..7cd12b0 100644
+--- a/flake8/utils.py
++++ b/flake8/utils.py
+@@ -75,8 +75,8 @@ def stdin_get_value():
+     return cached_value.getvalue()
+
+
+-def parse_unified_diff():
+-    # type: () -> List[str]
++def parse_unified_diff(diff=None):
++    # type: (str) -> List[str]
+     """Parse the unified diff passed on stdin.
+
+     :returns:
+@@ -84,7 +84,10 @@ def parse_unified_diff():
+     :rtype:
+         dict
+     """
+-    diff = stdin_get_value()
++    # Allow us to not have to patch out stdin_get_value
++    if diff is None:
++        diff = stdin_get_value()
++
+     number_of_rows = None
+     current_path = None
+     parsed_paths = collections.defaultdict(set)