Makefile: Git dependency, take 3

Makefile: Git dependency, take 3

In commit a1266edfe the build instructions for the git libs where moved
to their real targets, which in turn depended on the phony target `git`.
But since `git` is an actual directory in cgit the git libs wouldn't be
recompiled when needed.

So with this patch (third time lucky), cgit is declared to depend on the
really phony target `libgit` and the build instructions for `libgit` is
to unconditionally rebuild git/libgit.a and git/xdiff/lib.a.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>