worktree: give "should be pruned?" function more meaningful name

worktree: give "should be pruned?" function more meaningful name

Readers of the name prune_worktree() are likely to expect the function
to actually prune a worktree, however, it only answers the question
"should this worktree be pruned?". Give it a name more reflective of its
true purpose to avoid such confusion.

Signed-off-by: Eric Sunshine <redacted>
Signed-off-by: Junio C Hamano <redacted>