merge-recursive: clarify code in was_tracked()

merge-recursive: clarify code in was_tracked()

It can be puzzling to see that was_tracked() asks to get an index entry
by name, but does not take a negative return value for an answer.

The reason we have to do this is that cache_name_pos() only looks for
entries in stage 0, even if nobody asked for any stage in particular.

Let's rewrite the logic a little bit, to handle the easy case early: if
cache_name_pos() returned a non-negative position, we know it is a match,
and we do not even have to compare the name again (cache_name_pos() did
that for us already). We can say right away: yes, this file was tracked.

Only if there was no exact match do we need to look harder for any
matching entry in stage 2.

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