cat-file: simplify reading from standard input

cat-file: simplify reading from standard input

The batch modes of git-cat-file(1) read queries from stantard input that
are either newline- or NUL-delimited. This code was introduced via
db9d67f2e9 (builtin/cat-file.c: support NUL-delimited input with `-z`,
2022-07-22), which notes that:

"""
The refactoring here is slightly unfortunate, since we turn loops like:

     while (strbuf_getline(&buf, stdin) != EOF)

 into:

     while (1) {
         int ret;
         if (opt->nul_terminated)
             ret = strbuf_getline_nul(&input, stdin);
         else
             ret = strbuf_getline(&input, stdin);

         if (ret == EOF)
             break;
     }
"""

The commit proposed introducing a helper function that is easier to use,
which is just what we have done in the preceding commit. Refactor the
code to use this new helper to simplify the loop.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>