read-tree: fix eye-candy.

Anton Blanchard spotted that watching checkout stage of a clone
on a slow terminal takes ages because it forgot to clear the
"once a second happened" flag, so instead of updates the
percentage output for every file it checks out after the first
second has passed.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>