gcc-git: Add prepare-commit-msg hook.

gcc-git: Add prepare-commit-msg hook.

This patch introduces a prepare-commit-msg hook that appends a ChangeLog
skeleton to a commit message when the GCC_FORCE_MKLOG environment variable
is set, and a 'git commit-mklog' command set that variable while running
'git commit'.

contrib/ChangeLog:

	* prepare-commit-msg: New file.
	* gcc-git-customization.sh: Install it.  Add commit-mklog alias.
	* mklog.py: Add new option -c which appends
	to a ChangeLog file.