grep supports only author and committer headers, which have the same
special treatment that later headers may or may not have. Check for
field type and only strip_timestamp() when the field is either author
or committer.
GREP_HEADER_FIELD_MAX is put in the grep_header_field enum to be
calculated automatically, correctly, as long as it's at the end of the
enum.
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>