unidiff: use newline='\n' argument

In order to support CR on a line, we need to open files
with newline='\n' as our line endings supposed to be of UNIX style.

contrib/ChangeLog:

	* check_GNU_style.py: Use newline=\n.
	* check_GNU_style_lib.py: Simplify.
	* gcc-changelog/git_commit.py: Fix issues seen
	Rust patchset.
	* gcc-changelog/git_email.py: Use newline argument.
	* gcc-changelog/test_email.py: New test.
	* gcc-changelog/test_patches.txt: New test.
	* mklog.py: Use newline argument.