cat-file: add remove_timestamp helper

cat-file: add remove_timestamp helper

maybe_remove_timestamp() takes arguments, but it would be useful to have
a function that reads from stdin and strips the timestamp. This would
allow tests to pipe data into a function to remove timestamps, and
wouldn't have to always assign a variable. This is especially helpful
when the data is multiple lines.

Keep maybe_remove_timestamp() the same, but add a remove_timestamp
helper that reads from stdin.

The tests in the next patch will make use of this.

Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>