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 <43386ce32af96f5c56f2a88e458cb94cebee3751@gmail.com>
Signed-off-by: Jeff King <696e3fbcf235d40b6ea1ebd61c87cbea79d444e7@peff.net>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git