how to output absolute paths in git diff?

This has all the context:
https://stackoverflow.com/questions/22698505/how-to-show-full-paths-in-git-diff

I'd like `--show-abs-path` to show absolute paths in:
git diff --show-abs-path args...

eg:
git diff --no-index `get_file1` `get_file2`
could show:

--- a/Users/timothee/temp/ripgrep/help0.txt
+++ b/help1.txt

* passing '--dst-prefix=$PWD' and '--src-prefix=$PWD' doesn't help
because path arguments could be absolute, so it'll create
$PWD/Users/timothee/temp/ripgrep/help0.txt (wrong)

* passing '--dst-prefix=.' will behave weirdly, replacing leading `/`
by `.` (seems wrong)

diff --git .Users/timothee/temp/ripgrep/help0.txt b/help1.txt

NOTE: I'm invoking the `git diff` command via a more complicated case
(with multiple arguments including git diff flags and git diff files),
so it's awkward for me to parse which arguments correspond to a file
vs a flag (ie prevents easily converting input file arguments to
absolute paths), but `git` could do it easily via a flag, eg
`--show-abs-path`