merge-recursive: fix assumption that head tree being merged is HEAD

merge-recursive: fix assumption that head tree being merged is HEAD

`git merge-recursive` does a three-way merge between user-specified trees
base, head, and remote.  Since the user is allowed to specify head, we can
not necesarily assume that head == HEAD.

Modify index_has_changes() to take an extra argument specifying the tree
to compare against.  If NULL, it will compare to HEAD.  We then use this
from merge-recursive to make sure we compare to the user-specified head.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>