pretty: refactor `format_sanitized_subject()`

pretty: refactor `format_sanitized_subject()`

The function 'format_sanitized_subject()' is responsible for
sanitized subject line in pretty.c
e.g.
the subject line
the-sanitized-subject-line

It would be a nice enhancement to `subject` atom to have the
same feature. So in the later commits, we plan to add this feature
to ref-filter.

Refactor `format_sanitized_subject()`, so it can be reused in
ref-filter.c for adding new modifier `sanitize` to "subject" atom.

Currently, the loop inside `format_sanitized_subject()` runs
until `\n` is found. But now, we stored the first occurrence
of `\n` in a variable `eol` and passed it in
`format_sanitized_subject()`. And the loop runs upto `eol`.

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>