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 <jacknagel@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/subtree/Makefile

@@ -1,3 +1,6 @@
+# The default target of this Makefile is...
+all::
+
 -include ../../config.mak.autogen
 -include ../../config.mak
 
@@ -34,7 +37,7 @@ GIT_SUBTREE_XML := git-subtree.xml
 GIT_SUBTREE_TXT := git-subtree.txt
 GIT_SUBTREE_HTML := git-subtree.html
 
-all: $(GIT_SUBTREE)
+all:: $(GIT_SUBTREE)
 
 $(GIT_SUBTREE): $(GIT_SUBTREE_SH)
 	sed -e '1s|#!.*/sh|#!$(SHELL_PATH_SQ)|' $< >$@