gcc-changelog/git_commit.py: Check for missing description

Especially when using mklog.py, it is simply to forget to fill in
the entries after the '\t* file.c (section):' or '\t(section):'.

contrib/ChangeLog:

	* gcc-changelog/git_commit.py (item_parenthesis_empty_regex,
	item_parenthesis_regex): Add.
	(check_for_empty_description): Use them.
	* gcc-changelog/test_email.py (test_emptry_entry_desc,
	test_emptry_entry_desc_2): Add.
	* gcc-changelog/test_patches.txt: Add two testcases for it.