git: Update to 1.9.4

(cherry picked from commit 61befa04511551e3bdcb8150553e5c87bcd5abe3)
Manual: Note about using a local branch

(cherry picked from commit a8c9c11f9ec5b0d0d5eec279b66da84a89008f61)
qt-5: Don't build tests

Building the tests (which as far as I can tell we don't even run)
makes the build take > 19 GiB of disk space, which is a bit
excessive. Without the tests, it takes 2.6 GiB.

(cherry picked from commit 1cfea9bd1a61c7f30f0eee2c476e987dcacb882e)