doc: commit: link to git-status(1) on all format options

doc: commit: link to git-status(1) on all format options

`--branch` and `--long` refer to git-status(1) options but they don’t tell us
what `short-format` and `long-format` are, respectively. And `--null`
mentions “status” but does not link to the command.

Refer to git-config(1) on `--branch` like `--short` does.

`long-format` is the git-status(1) output. So we can just say that
directly.

Replace “status” with a `linkgit` on `--null`.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>