In Git, list names of branches with unpushed commits

git for-each-ref --format="%(refname:short) %(push:track)" refs/heads

That remain the most precise answer that you can easily parse/grep to get the desired output (like removing up-to-date branches)

You can do so in a bash script that you will call git-xxx (no extension), somewhere in your $PATH or %PATH%.
That script can then be called with git xxx, and will use git bash.
That is portable and will work across platforms (meaning even on Windows, where <Git For Windows>/usr/bin includes 200+ linux commands (grep, sed, awk, xargs, ...)

You can also see what branches are not yet merged to master

git checkout master

and then

git branch --no-merged

The --no-walk option to log seems to do a better job of what I need than --simplify-by-decoration. My full command is:

git log --branches --not --remotes --no-walk --decorate --oneline

...which I've aliased to unpushed.