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 <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

worktree.c

@@ -352,11 +352,11 @@ int is_worktree_being_rebased(const struct worktree *wt,
 
 	memset(&state, 0, sizeof(state));
 	found_rebase = wt_status_check_rebase(wt, &state) &&
-		((state.rebase_in_progress ||
-		  state.rebase_interactive_in_progress) &&
-		 state.branch &&
-		 starts_with(target, "refs/heads/") &&
-		 !strcmp(state.branch, target + strlen("refs/heads/")));
+		       (state.rebase_in_progress ||
+			state.rebase_interactive_in_progress) &&
+		       state.branch &&
+		       skip_prefix(target, "refs/heads/", &target) &&
+		       !strcmp(state.branch, target);
 	wt_status_state_free_buffers(&state);
 	return found_rebase;
 }
@@ -370,8 +370,8 @@ int is_worktree_being_bisected(const struct worktree *wt,
 	memset(&state, 0, sizeof(state));
 	found_bisect = wt_status_check_bisect(wt, &state) &&
 		       state.branch &&
-		       starts_with(target, "refs/heads/") &&
-		       !strcmp(state.branch, target + strlen("refs/heads/"));
+		       skip_prefix(target, "refs/heads/", &target) &&
+		       !strcmp(state.branch, target);
 	wt_status_state_free_buffers(&state);
 	return found_bisect;
 }