Move traversal of reachable objects into a separate library.

Move traversal of reachable objects into a separate library.

This moves major part of builtin-prune into a separate file,
reachable.c.  It is used to mark the objects that are reachable
from refs, and optionally from reflogs.

The patch looks very large, but if you look at it with diff -C,
which this message is formatted in, most of them are copied
lines and there are very little additions.

Signed-off-by: Junio C Hamano <junkio@cox.net>