Don’t use apt-get source; use dgit
tl;dr:
If you are a Debian user who knows git, don’t work with Debian source packages. Don’t use apt source, or dpkg-source. Instead, use dgit and work in git.
Also, don’t use: “VCS” links on official Debian web pages, debcheckout, or Debian’s (semi-)official gitlab, Salsa. These are suitable for Debian experts only; for most people they can be beartraps. Instead, use dgit.