The output from "git worktree list" was made in readdir() order,
and was unstable.
* nd/worktree-list-fixup:
worktree list: keep the list sorted
worktree.c: get_worktrees() takes a new flag argument
get_worktrees() must return main worktree as first item even on error
worktree: reorder an if statement
worktree.c: zero new 'struct worktree' on allocation