git-prune: never lose objects reachable from our refs.
Explicit <head> arguments to git-prune replaces, instead of
extends, the list of heads used for reachability analysis by
fsck-objects. By giving a subset of heads by mistake, objects
reachable only from other heads can be removed, resulting in a
corrupted repository.
This commit stops replacing the list of heads, and makes the
command line arguments to add to them instead for safety.