Custom git repositories and branches

This MR adds support for custom pmaports URLs for easy and fast local testing on VMs.

Changes to pmbootstrap.cfg:

[git_repos]
aports_upstream = https://gitlab.alpinelinux.org/alpine/aports.git
pmaports = https://gitlab.com/postmarketOS/pmaports.git

Custom branches can be downloaded by setting <git_url>#branch=<branch> in the URL.

Testing

Can be tested as pmbootstrap-custom-pmaports-git on AUR.