cherry-pick/revert: make direct internal call to merge_tree()

cherry-pick/revert: make direct internal call to merge_tree()

Refactored merge-recursive interface may still not be ideal but it already
allows us to make a direct call to merge_tree().

One regression is that the status message is lost as there is no way to
flush them from outside the refactored library code yet.

[jc: initial version by Miklos, with moderate amount of fixup by me]

Signed-off-by: Junio C Hamano <redacted>