The GPL requires that you distribute source in "the preferred form of the work for making modifications to it". If the workflow of programmers is heavily based on git, then the preferred form of the work is the git tree and that is what the GPL requires Red Hat (and others) to distribute. Flattening it into a tarball is no more acceptable than running it through a source mangler to strip all the comments. In both cases you no longer have the work in the form you prefer for modifying it.
Certainly, when Red Hat receive the code from Linus and pals it is in the form of a git tree, and if they distribute their changed version in a different form the onus is on them to show that this is now the "preferred" form for making modifications.