git-diff: fix missing --merge-base docs

When `git diff --merge-base` was introduced at around Git 2.30, the
documentation included a few errors.

In the example given for `git diff --cached --merge-base`, the
`--cached` flag was omitted for the `--merge-base` example. Add the
missing flag.

In the `git diff <commit>` case, we failed to mention that
`--merge-base` is an available option. Give the usage of `--merge-base`
as an option there.

Finally, there are two errors in the usage of `git diff`. Firstly, we do
not mention `--merge-base` in the `git diff --cached` case. Mention it
so that it's consistent with the documentation. Secondly, we put the
`[--merge-base]` in between `<commit>` and `[<commit>...]`. Move the
`[--merge-base]` so that it's beside `[<options>]` which is a more
logical grouping.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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