setuid and setgid with given user for git pull

[ryandv]

Resolves #758

Confirming that aura built from this revision can successfully pull mesa-git (the one I was struggling with).

This is quick and dirty to get the thing working, so let me know if anything needs to be touched up.

[fosskers]

Thank you for this - it's likely cleaner that sudoing down into the build user as I was going to do.

Note for anyone watching: this will not be an issue with Aura 4.

[fosskers]

Thanks again!