name-rev: use 'name->tip_name' instead of 'tip_name'

name-rev: use 'name->tip_name' instead of 'tip_name'

Following the previous patches in this series we can get the value of
'name_rev()'s 'tip_name' parameter from the 'struct rev_name'
associated with the commit as well.

So let's use 'name->tip_name' instead, which makes the patch
eliminating the recursion of name_rev() a bit easier to follow.

Note that at this point we could drop the 'tip_name' parameter as
well, but that parameter will be necessary later, after the recursion
is eliminated.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>