tree-diff: pass whole path string to path_appendnew()

tree-diff: pass whole path string to path_appendnew()

When diffing trees, we'll have a strbuf "base" containing the
slash-separted names of our parent trees, and a "path" string
representing an entry name from the current tree. We pass these
separately to path_appendnew(), which combines them to form a single
path string in the combine_diff_path struct.

Instead, let's append the path string to our base strbuf ourselves, pass
in the result, and then roll it back with strbuf_setlen(). This lets us
simplify path_appendnew() a bit, enabling further refactoring.

And while it might seem like this causes extra wasted allocations, it
does not in practice. We reuse the same strbuf for each tree entry, so
we only have to allocate it to match the largest name. Plus, in a
recursive diff we'll end up doing this same operation to extend the base
for the next level of recursion. So we're really just incurring a small
memcpy().

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