Miscellaneous const changes and utilities

Miscellaneous const changes and utilities

The list of remote refs in struct transport should be const, because
builtin-fetch will get confused if it changes.

The url in git_connect should be const (and work on a copy) instead of
requiring the caller to copy it.

match_refs doesn't modify the refspecs it gets.

get_fetch_map and get_remote_ref don't change the list they get.

Allow transport get_refs_list methods to modify the struct transport.

Add a function to copy a list of refs, when a function needs a mutable
copy of a const list.

Add a function to check the type of a ref, as per the code in connect.c

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>