tree-diff: don't assume compare_tree_entry() returns -1,0,1

tree-diff: don't assume compare_tree_entry() returns -1,0,1

It does, but we'll be reworking it in the next patch after it won't, and
besides it is better to stick to standard
strcmp/memcmp/base_name_compare/etc... convention, where comparison
function returns <0, =0, >0

Regarding performance, comparing for <0, =0, >0 should be a little bit
faster, than switch, because it is just 1 test-without-immediate
instruction and then up to 3 conditional branches, and in switch you
have up to 3 tests with immediate and up to 3 conditional branches.

No worry, that update_tree_entry(t2) is duplicated for =0 and >0 - it
will be good after we'll be adding support for multiparent walker and
will stay that way.

=0 case goes first, because it happens more often in real diffs - i.e.
paths are the same.

Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru>
Signed-off-by: Junio C Hamano <gitster@pobox.com>