apply: code simplification

apply: code simplification

Rewrite a bit hard-to-read ternary ?: expression into a cascade of
if/else.

Given that read-cache.c:add_index_entry() makes sure that the
.ce_mode member is filled with a reasonable value before placing a
cache entry in the index, if we see (ce_mode == 0), there is
something seriously wrong going on.  Catch such a bug and abort,
instead of silently ignoring such an entry and silently skipping
the check.

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