http: accept https:// proxies again

http: accept https:// proxies again

Since 663d7abe07ea (http: reject unsupported proxy URL schemes,
2026-05-05), set_curl_proxy_type() returns 0 only for the "http"
and SOCKS variants via dedicated early returns, and -1 for
everything else. The "https" branch configures the CURL handle for
HTTPS proxying but then falls through to the trailing `return -1`
intended for unknown schemes, so the caller in get_curl_handle()
treats a perfectly valid https:// proxy URL as unsupported and
refuses to use it.

Noticed while looking into a Coverity report against the same
function; the unchecked curl_easy_setopt() return values it flags
are orthogonal to this fix.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>