http.c: http.sslcert and http.sslkey are both pathnames

http.c: http.sslcert and http.sslkey are both pathnames

Back when the modern http_options() codepath was created to parse
various http.* options at 29508e1e ("Isolate shared HTTP request
functionality", 2005-11-18), and then later was corrected for
interation between the multiple configuration files in 7059cd99
("http_init(): Fix config file parsing", 2009-03-09), we parsed
configuration variables like http.sslkey, http.sslcert as plain
vanilla strings, because git_config_pathname() that understands
"~[username]/" prefix did not exist.  Later, we converted some of
them (namely, http.sslCAPath and http.sslCAInfo) to use the
function, and added variables like http.cookeyFile http.pinnedpubkey
to use the function from the beginning.  Because of that, these
variables all understand "~[username]/" prefix.

Make the remaining two variables, http.sslcert and http.sslkey, also
aware of the convention, as they are both clearly pathnames to
files.

Noticed-by: Victor Toni <victor.toni@gmail.com>
Helped-by: Charles Bailey <cbailey32@bloomberg.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>