worktree.c: check whether branch is bisected in another worktree

Similar to the rebase case, we want to detect if "HEAD" in some worktree
is being bisected because

1) we do not want to checkout this branch in another worktree, after
   bisect is done it will want to go back to this branch

2) we do not want to delete the branch is either or git bisect will
   fail to return to the (long gone) branch

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t2025-worktree-add.sh

@@ -263,4 +263,17 @@ test_expect_success 'check out from current worktree branch ok' '
 	)
 '
 
+test_expect_success 'checkout a branch under bisect' '
+	git worktree add under-bisect &&
+	(
+		cd under-bisect &&
+		git bisect start &&
+		git bisect bad &&
+		git bisect good HEAD~2 &&
+		git worktree list | grep "under-bisect.*detached HEAD" &&
+		test_must_fail git worktree add new-bisect under-bisect &&
+		! test -d new-bisect
+	)
+'
+
 test_done

worktree.c

@@ -234,6 +234,21 @@ static int is_worktree_being_rebased(const struct worktree *wt,
 	return found_rebase;
 }
 
+static int is_worktree_being_bisected(const struct worktree *wt,
+				      const char *target)
+{
+	struct wt_status_state state;
+	int found_rebase;
+
+	memset(&state, 0, sizeof(state));
+	found_rebase = wt_status_check_bisect(wt, &state) &&
+		state.branch &&
+		starts_with(target, "refs/heads/") &&
+		!strcmp(state.branch, target + strlen("refs/heads/"));
+	free(state.branch);
+	return found_rebase;
+}
+
 /*
  * note: this function should be able to detect shared symref even if
  * HEAD is temporarily detached (e.g. in the middle of rebase or
@@ -261,6 +276,10 @@ const struct worktree *find_shared_symref(const char *symref,
 				existing = wt;
 				break;
 			}
+			if (is_worktree_being_bisected(wt, target)) {
+				existing = wt;
+				break;
+			}
 		}
 
 		strbuf_reset(&path);