Don't yap about merge-subtree during make

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: Shawn O. Pearce <redacted>
Signed-off-by: Junio C Hamano <redacted>