http: add const to fix strchr() warnings

http: add const to fix strchr() warnings

The "path" field of a "struct repo" (a custom http-push struct, not to
be confused with "struct repository) is a pointer into a const argv
string, and is never written to.

The compiler does not traditionally complain about assigning from a
const pointer because it happens via strchr(). But with some C23 libc
versions (notably recent glibc), it has started to do so. Let's mark the
field as const to silence the warnings.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>