Makefile: fix parallel "make test"

When building the "test" target we depend on both cgit and building the
Git tools.  By doing this with two targets we end up running make in the
git/ directory twice, concurrently if using parallel make, which causes
us to build more than we need and potentially builds incorrectly if
multi-step build-then-move operations overlap.

Fix this by instead calling back into the makefile so that we alter the
"cgit" target to also build the Git tools.

Signed-off-by: John Keeping <john@keeping.me.uk>