tree-diff: simplify emit_path() list management

tree-diff: simplify emit_path() list management

In emit_path() we may append a new combine_diff_path entry to our list,
decide that we don't want it (because opt->pathchange() told us so) and
then roll it back.

Between the addition and the rollback, it doesn't matter if it's in the
list or not (no functions can even tell, since it's a singly-linked list
and we pass around just the tail entry).

So it's much simpler to just wait until opt->pathchange() tells us
whether to keep it, and either attach it (or free it) then. We do still
have to allocate it up front since it's that struct itself which is
passed to the pathchange callback.

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