Add support for upstream repos

[alexconst]

zgen.zsh allows the user to use his own prezto and OMZ forks by using ZGEN_PREZTO_REPO and ZGEN_OH_MY_ZSH_REPO. But then you have a problem with upstream, where you have to manually add it and update it.

It would be nice to have some sort of mechanism, to keep track and update from upstream. Maybe having these ZGEN_PREZTO_REPO_UPSTREAM=sorin-ionescu and ZGEN_OH_MY_ZSH_REPO_UPSTREAM=robbyrussell variables defined in zgen.zsh. If the user overrides them (to empty string or undefines them), then updates would be done from remote origin forks. Otherwise from remote upstream.

[tarjoilija]

It would be nice to have some sort of mechanism, to keep track and update from upstream.

I'm not completely sure what you're suggesting here.

So you have your own local oh-my-zsh fork that has robbyrussell/oh-my-zsh set as a upstream repo? I don't think there's any proper way to magically merge two repositories as they may have merge conflicts.