combine-diff: speed it up, by using multiparent diff tree-walker directly

As was recently shown in "combine-diff: optimize
combine_diff_path sets intersection", combine-diff runs very slowly. In
that commit we optimized paths sets intersection, but that accounted
only for ~ 25% of the slowness, and as my tracing showed, for linux.git
v3.10..v3.11, for merges a lot of time is spent computing
diff(commit,commit^2) just to only then intersect that huge diff to
almost small set of files from diff(commit,commit^1).

In previous commit, we described the problem in more details, and
reworked the diff tree-walker to be general one - i.e. to work in
multiple parent case too. Now is the time to take advantage of it for
finding paths for combine diff.

The implementation is straightforward - if we know, we can get generated
diff paths directly, and at present that means no diff filtering or
rename/copy detection was requested(*), we can call multiparent tree-walker
directly and get ready paths.

(*) because e.g. at present, all diffcore transformations work on
    diff_filepair queues, but in the future, that limitation can be
    lifted, if filters would operate directly on combine_diff_paths.

Timings for `git log --raw --no-abbrev --no-renames` without `-c` ("git log")
and with `-c` ("git log -c") and with `-c --merges` ("git log -c --merges")
before and after the patch are as follows:

                linux.git v3.10..v3.11

            log     log -c     log -c --merges

    before  1.9s    16.4s      15.2s
    after   1.9s     2.4s       1.1s

The result stayed the same.

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

diff.c

@@ -4764,6 +4764,7 @@ void diffcore_fix_diff_index(struct diff_options *options)
 
 void diffcore_std(struct diff_options *options)
 {
+	/* NOTE please keep the following in sync with diff_tree_combined() */
 	if (options->skip_stat_unmatch)
 		diffcore_skip_stat_unmatch(options);
 	if (!options->found_follow) {