sequencer: lib'ify do_recursive_merge()

sequencer: lib'ify do_recursive_merge()

Instead of dying there, let the caller high up in the callchain
notice the error and handle it (by dying, still).

The only caller of do_recursive_merge(), do_pick_commit() already
checks the return value and passes it on to its callers, so its caller
must be already prepared to handle error returns, and with this step,
we make it notice an error return from this function.

So this is a safe conversion to make do_recursive_merge() callable
from new callers that want it not to die, without changing the
external behaviour of anything existing.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>