build: Update git.mk from upstream
build: Update git.mk from upstream

Upstream is now located at https://github.com/behdad/git.mk. The file hasn’t
been modified from upstream.

renatofilho/folks2,GNOME/folks,GNOME/folks,renatofilho/folks,renatofilho/folks,pwithnall/folks,renatofilho/folks2,GNOME/folks,pwithnall/folks,renatofilho/folks2,pwithnall/folks,renatofilho/folks2,renatofilho/folks,renatofilho/folks