Fix various dead stores found by the clang static analyzer

Fix various dead stores found by the clang static analyzer

http-push.c::finish_request():
  request is initialized by the for loop

index-pack.c::free_base_data():
  b is initialized by the for loop

merge-recursive.c::process_renames():
  move compare to narrower scope, and remove unused assignments to it
  remove unused variable renames2

xdiff/xdiffi.c::xdl_recs_cmp():
  remove unused variable ec

xdiff/xemit.c::xdl_emit_diff():
  xche is always overwritten

Signed-off-by: Benjamin Kramer <benny.kra@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>