subtree: make "all" default target of Makefile

subtree: make "all" default target of Makefile

You should be able to run "make" in contrib/subtree with no
arguments and get the "all" target. This was broken by 8e2a5cc
(contrib/subtree/Makefile: use GIT-VERSION-FILE, 2014-05-06), which
put the rule for GIT-VERSION-FILE higher in the file.

We can fix this by putting an empty "all::" target at the top of the
file, just like our main Makefile does, and document that fact.
That fixes this instance and future-proofs against it happening
again.

Reported-by: Jack Nagel <redacted>
Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>