Make bats-file safe to use under -u

[bats-core]

Make bats-file safe to use under -u

[bkahlert]

@srcbucket you can use the following snippet in the mean time. It wraps all assert_file_ calls with code that makes sure the two variables in question are set.

      local bats_fn bats_decl
      for bats_fn in $(set | grep -e "^assert_file_*"); do
        [[ $bats_fn == assert_file_* ]] || continue
        bats_decl=$(declare -f "$bats_fn") || true
        if [ "${bats_decl-}" ]; then
          eval "bats_$bats_decl"
          eval "$bats_fn() {
            BATSLIB_FILE_PATH_REM=\${BATSLIB_FILE_PATH_REM:-} \
            BATSLIB_FILE_PATH_ADD=\${BATSLIB_FILE_PATH_ADD:-} \
            bats_$bats_fn \"\$@\"
          }"
        fi
      done