Merge branch 'bc/zsh-compatibility'

zsh can pretend to be a normal shell pretty well except for some
glitches that we tickle in some of our scripts. Work them around
so that "vimdiff" and our test suite works well enough with it.

* bc/zsh-compatibility:
  vimdiff: make script and tests work with zsh
  t4046: avoid continue in &&-chain for zsh