Factor out a dwim_branch function, which takes care of the dwim'ery in
'git worktree add <path>'.  It's not too much code currently, but we're
adding a new kind of dwim in a subsequent patch, at which point it makes
more sense to have it as a separate function.
[...]
Signed-off-by: Thomas Gummerer <redacted>
---

diff --git a/builtin/worktree.c b/builtin/worktree.c

@@ -417,16 +431,9 @@ static int add(int ac, const char **av, const char *prefix)
        if (ac < 2 && !opts.new_branch && !opts.detach) {
-               int n;
-               const char *s = worktree_basename(path, &n);
-               opts.new_branch = xstrndup(s, n);
-               if (guess_remote) {
-                       struct object_id oid;
-                       const char *remote =
-                               unique_tracking_name(opts.new_branch, &oid);
-                       if (remote)
-                               branch = remote;
-               }
+               const char *dwim_branchname = dwim_branch(path, &opts);
+               if (dwim_branchname)
+                       branch = dwim_branchname;