mirrors.bbclass: provide additional rule for git repo fallbacks

Git hosting servers that require a slightly different URL when accessing
repositories via https protocol instead of the git native protocol
are not uncommon (servers using cgit as web UI). Provide a general rule
to try HOST/git/PATH via https as git repo fallback.

Signed-off-by: Daniel Wagenknecht <dwagenknecht@emlix.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>