Generate better mtimes.  (Still room for improvement.)

In particular, some things (eg, HEAD-file) are generated with a zero
mtimes even though in principle they should have real mtimes.

As of this commit, the list is:
	HEAD-file
	tag-file/$TAGNAME
	branch-file/$BRANCHNAME
	abbrev-file/$ABBREV
	commit/$COMMIT/parents-file/$N