mklog: add subject line skeleton

In the recent gcc-commit-mklog thread on gcc@ it occurred to me that the
command could also fill in part of the subject line.  If the first PR is
foo/1234, and the commit does not yet have a subject line, this will add

foo: [PR1234]


	* Add an initial component: [PRnnnnn] line when
	we have a PR.
1 file changed