apply: pass apply state to build_fake_ancestor()

apply: pass apply state to build_fake_ancestor()

To libify git apply functionality, we will need to read from a
different index file in get_current_sha1(). This index file will be
stored in "struct apply_state", so let's pass the state to
build_fake_ancestor() which will later pass it to get_current_sha1().

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