.gitconfig

[alias]
  outdated = "!f() { bash -c 'diff -u <(git branch --no-merged master) <(git branch --contains master) | egrep \"^- \"'; }; f"
  update = "!f() { set -e; git checkout $1; git rebase master; git push -f origin $1; git checkout -; }; f"

example.sh

> git outdated
- some-pr-branch-not-up-to-date-with-master

> git update some-old-branch
(check out branch)
(rebase master)
(force push branch)