get_patch_filename(): simplify function signature

get_patch_filename(): simplify function signature

Most functions that emit to a strbuf take the strbuf as their first
parameter; make this function follow suit.

The serial number of the patch being emitted (nr) and suffix used
for patch filename (suffix) are both recorded in rev_info; drop
these separate parameters and pass the rev_info directly.

Signed-off-by: Junio C Hamano <redacted>