help: cleanup the contruction of keys_uniq

help: cleanup the contruction of keys_uniq

construction of keys_uniq depends on sort operation
executed on keys before processing, which does not
gurantee that keys_uniq will be sorted.

refactor the code to shift the sort operation after
the processing to remove dependency on key's sort operation
and strictly maintain the sorted order of keys_uniq.

move strbuf init and release out of loop to reuse same buffer.

dedent sort -u and sed in tests and replace grep with sed, to
avoid piping grep's output to sed.

Suggested-by: Siddharth Shrimali <r.siddharth.shrimali@gmail.com>
Signed-off-by: Amisha Chhajed <amishhhaaaa@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>