Zig requires alignment to be specified when specifying a fill character,
as otherwise digits specified after ':' are interpreted as part of the
field width.

The missing alignment specifier caused character codes < 0x10 to be
serialized incorrectly, producing an export file ncdu could not import.

For example, a character with code 1 would be serialized as '\u00 1'
instead of '\u0001'.

A directory of test files can be generated using:

    mkdir test_files; i=1; while [ $i -le 255 ]; do c="$(printf "$(printf "\\\\x%02xZ" "$i")")"; c="${c%Z}"; touch "test_files/$c"; i=$((i+1)); done