ref-filter: add `sanitize` option for 'subject' atom

ref-filter: add `sanitize` option for 'subject' atom

Currently, subject does not take any arguments. This commit introduce
`sanitize` formatting option to 'subject' atom.

`subject:sanitize` - print sanitized subject line, suitable for a filename.

e.g.
%(subject): "the subject line"
%(subject:sanitize): "the-subject-line"

Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Heba Waly <heba.waly@gmail.com>
Signed-off-by: Hariom Verma <hariom18599@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>