merge: clarify call chain

prepare_to_commit() cannot be reached in the non-squash case:
It is called by merge_trivial() and finish_automerge() only, but the
calls to the latter are somewhat hard to track:

If option_commit is not set, the code in cmd_merge() uses a fake
conflict return code (ret=1) to avoid writing the tree, which also
avoids setting automerge_was_ok (just as in the proper ret==1 case), so
that finish_automerge() is not called.

To ensure that no code change breaks that assumption, safe-guard
prepare_to_commit() by a BUG() statement.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Michael J Gruber <git@grubix.eu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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