t/t4013/diff.log_--diff-merges=first-parent_main

$ git log --diff-merges=first-parent main
commit 59d314ad6f356dd08601a4cd5e530381da3e3c64
Merge: 9a6d494 c7a2ab9
Author: A U Thor <author@example.com>
Date:   Mon Jun 26 00:04:00 2006 +0000

    Merge branch 'side'

diff --git a/dir/sub b/dir/sub
index cead32e..992913c 100644
--- a/dir/sub
+++ b/dir/sub
@@ -4,3 +4,5 @@ C
 D
 E
 F
+1
+2
diff --git a/file0 b/file0
index b414108..10a8a9f 100644
--- a/file0
+++ b/file0
@@ -4,3 +4,6 @@
 4
 5
 6
+A
+B
+C

commit c7a2ab9e8eac7b117442a607d5a9b3950ae34d5a
Author: A U Thor <author@example.com>
Date:   Mon Jun 26 00:03:00 2006 +0000

    Side

commit 9a6d4949b6b76956d9d5e26f2791ec2ceff5fdc0
Author: A U Thor <author@example.com>
Date:   Mon Jun 26 00:02:00 2006 +0000

    Third

commit 1bde4ae5f36c8d9abe3a0fce0c6aab3c4a12fe44
Author: A U Thor <author@example.com>
Date:   Mon Jun 26 00:01:00 2006 +0000

    Second
    
    This is the second commit.

commit 444ac553ac7612cc88969031b02b3767fb8a353a
Author: A U Thor <author@example.com>
Date:   Mon Jun 26 00:00:00 2006 +0000

    Initial
$