Git alias with positional parameters

A shell function could help on this:

[alias]
    files = "!f() { git diff --name-status \"$1^\" \"$1\"; }; f"

An alias without ! is treated as a Git command; e.g. commit-all = commit -a.

With the !, it's run as its own command in the shell, letting you use stronger magic like this.

UPD
Because commands are executed at the root of repository you may use ${GIT_PREFIX} variable when referring to the file names in commands

You can also reference sh directly (instead of creating a function):

[alias]
        files = !sh -c 'git diff --name-status $1^ $1' -

(Note the dash at the end of the line -- you'll need that.)

The alias you are looking for is:

files = "!git diff --name-status \"$1\"^ \"$1\" #"

With argument validation:

files = "!cd -- \"${GIT_PREFIX:-.}\" && [ x$# != x1 ] && echo commit-ish required >&2 || git diff --name-status \"$1\"^ \"$1\" #"

The final # is important - it prevents all the user-supplied arguments from being processed by the shell (it comments them out).

Note: git puts all user-supplied arguments at the end of the command line. To see this in action, try: GIT_TRACE=2 git files a b c d

The escaped (due to nesting) quotes are important for filenames containing spaces or "; rm -rf --no-preserve-root /;)