gitfast: update to upstream v2.21 (#7914)

* gitfast: use $OSTYPE again

In the last update to upstream this was reverted:

 a56eac7a (Use OSTYPE instead of uname whenever possible for better speed. (#5496))

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>

* gitfast: simplify plugin

No need to set and unset a variable we use once.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>

* gitfast: add script to update from upstream

This would make easier the process of updating, and also not miss our
patches.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>

* gitfast: update to upstream v2.21

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>