Merge branch 'mt/cleanly-die-upon-missing-required-filter'

We had a code to diagnose and die cleanly when a required
clean/smudge filter is missing, but an assert before that
unnecessarily fired, hiding the end-user facing die() message.

* mt/cleanly-die-upon-missing-required-filter:
  convert: fail gracefully upon missing clean cmd on required filter

diff --cc convert.c
Simple merge

diff --cc t/t0021-conversion.sh
Simple merge