Add `--relative` to some `git diff` alia
Add `--relative` to some `git diff` alia

`gd`, `gdn`, and `gdcn` will now be relative.

Kind of annoying that alias expansion isn't recursive, so can't use a
"base" `diff` alias.

Changing `dn = diff --name-only` to `dn = "!git d --name-only"` loses
the "relative"-ness, since shell-expansion `cd`s into the git root
directory first.  I don't want to have to prepend `!eval `git cd` ;` to
all my diff-related alia the same way I did with my check&&add alia.

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