column: fix parsing of the '--nl' option

'git column's '--nl' option can be used to specify a "string to be printed at the end of each line" (quoting the man page), but this option and its mandatory argument has been parsed as OPT_INTEGER since the introduction of the command in 7e29b8254f (Add column layout skeleton and git-column, 2012-04-21). Consequently, any non-number argument is rejected by parse-options, and any number other than 0 leads to segfault: $ printf "%s\n" one two |git column --mode=plain --nl=foo error: option `nl' expects a numerical value $ printf "%s\n" one two |git column --mode=plain --nl=42 Segmentation fault (core dumped) $ printf "%s\n" one two |git column --mode=plain --nl=0 one two Parse this option as OPT_STRING. Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>