scripts/bash_aliases: man_gitstaged(): Script around a porcelain git command

scripts/bash_aliases: man_gitstaged(): Script around a porcelain git command

The output of 'git status' is not stable.

The more stable 'git status --porcelain' is more complex,
and scripting around it would be more complex.

However, 'git diff --staged --name-only' produces
the output that we were lookiong for.

Reported-by: Jakub Wilk <jwilk@jwilk.net>
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>