contrib/gcc-changelog: Detect if same file appears twice

contrib/ChangeLog:

	* gcc-changelog/git_commit.py (Error.__repr__): Add space after the colon.
	(GitCommit.check_mentioned_files): Check whether the same file has been
	specified multiple times.
	* gcc-changelog/test_email.py (TestGccChangelog.test_multi_same_file): New.
	* gcc-changelog/test_patches.txt (0001-OpenMP-Fix-SIMT): New test.