Display the local paths from the git hook

Instead of displaying the path to the temporary directory, as we always
have, it's been requested that we instead display the path to the file
that was copied to the temporary directory.

Closes #244

src/flake8/main/git.py

@@ -6,6 +6,7 @@
 """
 import contextlib
 import os
+import os.path
 import shutil
 import stat
 import subprocess
@@ -46,6 +47,7 @@ def hook(lazy=False, strict=False):
         app.options._running_from_vcs = True
         app.run_checks(filepaths)
 
+    update_paths(app.file_checker_manager, tempdir)
     app.report_errors()
     if strict:
         return app.result_count
@@ -204,6 +206,16 @@ def update_excludes(exclude_list, temporary_directory_path):
     ]
 
 
+def update_paths(checker_manager, temp_prefix):
+    temp_prefix_length = len(temp_prefix)
+    for checker in checker_manager.checkers:
+        filename = checker.display_name
+        if filename.startswith(temp_prefix):
+            checker.display_name = os.path.relpath(
+                filename[temp_prefix_length:]
+            )
+
+
 _HOOK_TEMPLATE = """#!{executable}
 import os
 import sys