pass struct commit to diff_tree_combined_merge()

Instead of passing the hash of a commit and then searching that
same commit in the single caller, simply pass the commit directly.

Signed-off-by: René Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

log-tree.c

diff --git a/log-tree.c b/log-tree.c
index e7694a3..319bd31 100644
--- a/log-tree.c
+++ b/log-tree.c
@@ -599,9 +599,7 @@
 
 static int do_diff_combined(struct rev_info *opt, struct commit *commit)
 {
-	unsigned const char *sha1 = commit->object.sha1;
-
-	diff_tree_combined_merge(sha1, opt->dense_combined_merges, opt);
+	diff_tree_combined_merge(commit, opt->dense_combined_merges, opt);
 	return !opt->loginfo;
 }