merge-recursive.c: mrtree in merge() is not used before set

merge-recursive.c: mrtree in merge() is not used before set

The called function merge_trees() sets its *result, to which the
address of the variable mrtree in merge() function is passed,
only when index_only is set.  But that is Ok as the function
uses the value in the variable only under index_only iteration.

However, recent gcc does not realize this.  Work it around by
adding a fake initializer.

Signed-off-by: Junio C Hamano <gitster@pobox.com>