match-trees: compute buffer offset correctly when splicing

match-trees: compute buffer offset correctly when splicing

Currently, the struct object_id pointer returned from tree_entry_extract
lives directly inside the parsed tree buffer. In a future commit, this
will change so that it instead points to a dedicated struct member.
Since in this code path, we want to modify the buffer directly, compute
the buffer offset we want to modify by using the pointer to the path
instead.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>