Remove git push config

- it doesn't work for older versions of git
- it's the default in newer versions of git
- it doesn't matter if you use `push origin HEAD` (the `put` alias in
  this file)
Add git amend alias
Git: cache credentials

- default is for 15 minutes
- see https://www.kernel.org/pub/software/scm/git/docs/git-credential-cache.html
  for more info
Add vanilla emacs aliases
Add aliases to open multiple files in splits/tabs