Like calloc(3), xcalloc() takes nmemb and then size.

* bg/xcalloc-nmemb-then-size:
  transport-helper.c: rearrange xcalloc arguments
  remote.c: rearrange xcalloc arguments
  reflog-walk.c: rearrange xcalloc arguments
  pack-revindex.c: rearrange xcalloc arguments
  notes.c: rearrange xcalloc arguments
  imap-send.c: rearrange xcalloc arguments
  http-push.c: rearrange xcalloc arguments
  diff.c: rearrange xcalloc arguments
  config.c: rearrange xcalloc arguments
  commit.c: rearrange xcalloc arguments
  builtin/remote.c: rearrange xcalloc arguments
  builtin/ls-remote.c: rearrange xcalloc arguments