Makefile: another take on git dependency rules

Makefile: another take on git dependency rules

When building cgit we depend on xdiff/lib.a and libgit.a in the git
directory, but the previous attempt on describing this dependency
failed since the build instructions for the libs was placed under the
phony `git` target.

This patch fixes the issue by moving the build instructions to their
real targets. It also makes it clear that only the `cgit` target
depends on the git binaries (since they're only used during linking).

And while at it, the patch also cleans up the list of phony targets.

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