connect: simplify check_ref() using skip_prefix() and starts_with()

connect: simplify check_ref() using skip_prefix() and starts_with()

Both callers of check_ref() pass in NUL-terminated strings for name.
Remove the len parameter and then use skip_prefix() and starts_with()
instead of memcmp() to check if it starts with certain strings.  This
gets rid of several magic string length constants and a strlen() call.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>