gcc-changelog: handle entries with multi-line file lists

This extends the ChangeLog entries parsing machinery to handle entries
that cover multiple files spanning over multiple lines. For instance:

    * first_file_patched.c, second_file_patched.c, third_file_patched.c,
    fourth_file_patched.c: Do things.

contrib/

	* gcc-changelog/git_commit.py (ChangeLogEntry): Handle entries
	with multi-line file lists.
	* gcc-changelog/test_email.py: New testcase.
	* gcc-changelog/test_patches.txt: Likewise.