convert: add const to fix strchr() warnings

convert: add const to fix strchr() warnings

C23 versions of libc (like recent glibc) may provide generic versions of
strchr() that match constness between the input and return value. The
idea being that the compiler can detect when it implicitly converts a
const pointer into a non-const one (which then emits a warning).

There are a few cases here where the result pointer does not need to be
non-const at all, and we should mark it as such. That silences the
warning (and avoids any potential problems with trying to write via
those pointers).

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