diffcore-rename: avoid unnecessary strdup'ing in break_idx

diffcore-rename: avoid unnecessary strdup'ing in break_idx

The keys of break_idx are strings from the diff_filepairs of
diff_queued_diff.  break_idx is only used in location_rename_dst(), and
that usage is always before any free'ing of the pairs (and thus the
strings in the pairs).  As such, there is no need to strdup these keys;
we can just reuse the existing strings as-is.

The merge logic doesn't make use of break detection, so this does not
affect the performance of any of my testcases.  It was just a minor
unrelated optimization noted in passing while looking at the code.

Signed-off-by: Elijah Newren <newren@gmail.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>