update git cheat sheet (master -> main, merge -> rebase)

Updates the git cheat sheet (pdf) as suggested by @fwinkl in #45 (closed)

The pdf is created from a keynote file located in my (wlampl's) cernbox: https://cernbox.cern.ch/s/giZ6877x3QmsxTb (I realize that keynote is a propriatory format not accessible to everyone)