Correct exclude paths in git hook

When running the git hook, even though the appropriate config file is
found and processed, the exclude patterns and paths were not being
corrected to account for the temporary directory that we copy the files
into.

Related-to #223

src/flake8/main/git.py

@@ -42,6 +42,7 @@ def hook(lazy=False, strict=False):
     with make_temporary_directory() as tempdir:
         filepaths = list(copy_indexed_files_to(tempdir, lazy))
         app.initialize(['.'])
+        app.options.exclude = update_excludes(app.options.exclude, tempdir)
         app.run_checks(filepaths)
 
     app.report_errors()
@@ -194,6 +195,14 @@ def config_for(parameter):
     return value.lower() in defaults.TRUTHY_VALUES
 
 
+def update_excludes(exclude_list, temporary_directory_path):
+    return [
+        (temporary_directory_path + pattern)
+        if os.path.isabs(pattern) else pattern
+        for pattern in exclude_list
+    ]
+
+
 _HOOK_TEMPLATE = """#!{executable}
 import os
 import sys