Export parse_date_basic() to convert a date string to timestamp

Export parse_date_basic() to convert a date string to timestamp

approxidate() is not appropriate for reading machine-written dates
because it guesses instead of erroring out on malformed dates.
parse_date() is less convenient since it returns its output as a
string.  So export the underlying function that writes a timestamp.

While at it, change the return value to match the usual convention:
return 0 for success and -1 for failure.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>