Remove a legacy code path from back in the day, where pmaports were in
the same repository as pmbootstrap ([1]). Nowadays, pmaports are always in a
git repository, so this is not necessary anymore. If git fails at this
point, crash hard and don't carry on with an empty string.
This is in preparation for using rev_parse in new code paths, so we
don't need to check the return value of rev_parse.
[1] 39548835 "Write custom os-release (closes#324) (#439)"
https://github.com/postmarketOS/pmbootstrap/pull/439
2b1dfe7cfc