My bashrc file sources the git prompt helper to show the branch I'm on in the prompt. Unfortunately it's quite old and was pointing to the wrong file, how do I find where it is?

dpkg -L git | grep prompt

Debian and its derivatives such as Ubuntu you can use apt to manage packages (e.g. apt upgrade, apt install). However apt is just a thin layer over dpkg that does useful things like resolving dependencies and downloading files. If you want to get low level with packages in Debian derivative distriubutions, look into dpkg.

From the man page --listfiles or equivalently -L lists all files installed from a package. So dpkg -L git shows all files installed in the git package, including the prompt. We can then use grep to filter the results.

In Arch Linux it's pacman -Ql git. For systems such as Fedora using yum it's repoquery -l git.