rerere: stop looping unnecessarily

rerere: stop looping unnecessarily

handle_cache() loops 3 times starting from an index entry that is
unmerged, while ignoring an entry for a path that is different from
what we are looking for.

As the index is sorted, once we see a different path, we know we saw
all stages for the path we are interested in.  Just loop while we
see the same path and then break, instead of continuing for 3 times.

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