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>