hacking-howto: Update git section
hacking-howto: Update git section

This still had some leftovers from the patch era. Since git and
specifically GitHub-like developing is much more mainstream now we don't
need to link introductions or even mention the idea of "patching".

The deleted "them" was referencing an old sentence referring to patches,
generated by the git cli. As emailing patches is not common at all for
GitHub repos, I removed the sentence altogether.

Also simplifies the 'branches' subsection a bit. Asking people to verify
their patch on master seems too much preliminary work and I doubt that
anyone does it anyway.

Airblader/i3,Airblader/i3,stapelberg/i3,stapelberg/i3,Airblader/i3,i3/i3,i3/i3,Airblader/i3-original,Airblader/i3-original,Airblader/i3-original,Airblader/i3,Airblader/i3-original,stapelberg/i3,i3/i3,i3/i3,stapelberg/i3,Airblader/i3-original