gcc-changelog: workaround for utf8 filenames

gcc-changelog: workaround for utf8 filenames

contrib/ChangeLog:

	* gcc-changelog/git_commit.py: Add decode_path function.
	* gcc-changelog/git_email.py: Use it in order to solve
	utf8 encoding filename issues.
	* gcc-changelog/git_repository.py: Likewise.
	* gcc-changelog/test_email.py: Test it.