Don't yap about merge-subtree during make

By default we are pretty quiet about the actual commands that
we are running.  So we should continue to be quiet about the new
merge-subtree hardlink to merge-recursive.  Technically this is not
a builtin, but it is close because subtree is actually builtin to
a non-builtin.  So lets just make things easy and call it a builtin.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Makefile

@@ -728,7 +728,7 @@ git$X: git.c common-cmds.h $(BUILTIN_OBJS) $(GITLIBS) GIT-CFLAGS
 help.o: common-cmds.h
 
 git-merge-subtree$X: git-merge-recursive$X
-	rm -f $@ && ln git-merge-recursive$X $@
+	$(QUIET_BUILT_IN)rm -f $@ && ln git-merge-recursive$X $@
 
 $(BUILT_INS): git$X
 	$(QUIET_BUILT_IN)rm -f $@ && ln git$X $@