Re: [PATCH 7/8] worktree: generalize candidate worktree path validation

Eric Sunshine [off-list ref] writes:

"git worktree add" checks that the specified path is a valid location
for a new worktree by ensuring that the path does not already exist and
is not already registered to another worktree (a path can be registered
but missing, for instance, if it resides on removable media). Since "git
worktree add" is not the only command which should perform such
validation ("git worktree move" ought to also), generalize the the
validation function for use by other callers, as well.

Makes sense.

-	validate_worktree_add(path, opts);
+	worktrees = get_worktrees(0);
+	check_candidate_path(path, opts->force, worktrees, "add");
+	free_worktrees(worktrees);
+	worktrees = NULL;

It is somewhat unsatisfying that the libified helper still signals
its displeasure by dying, but a faithful conversion that can be
cleaned up later (if we wanted to) like this step is easier to
reason about.