worktree: use skip_prefix to parse target

worktree: use skip_prefix to parse target

Instead of checking for "refs/heads/" using `starts_with()`, then
skipping past "refs/heads/" using `strlen()`, just use `skip_prefix()`.

In `is_worktree_being_rebased()`, we can adjust the indentation while
we're here and lose a pair of parentheses which isn't needed and which
might even make the reader wonder what they're missing and why that
grouping is there.

Signed-off-by: Martin Ågren <redacted>
Signed-off-by: Junio C Hamano <redacted>