t4013: add tests for --diff-merges=first-parent

This new option provides essential new functionality, changing diff
output to first parent only, without changing history traversal mode,
so it deserves its own test.

As we do it, add additional test that --diff-merges=first-parent by
itself doesn't imply -p and only outputs diffs for merge commits.

Signed-off-by: Sergey Organov <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: sorganov

t/t4013-diff-various.sh

@@ -327,6 +327,8 @@ log --no-diff-merges -p --first-parent master
 log --diff-merges=off -p --first-parent master
 log --first-parent --diff-merges=off -p master
 log -p --first-parent master
+log -p --diff-merges=first-parent master
+log --diff-merges=first-parent master
 log -m -p --first-parent master
 log -m -p master
 log --cc -m -p master

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

@@ -0,0 +1,56 @@
+$ git log --diff-merges=first-parent master
+commit 59d314ad6f356dd08601a4cd5e530381da3e3c64
+Merge: 9a6d494 c7a2ab9
+Author: A U Thor <[email protected]>
+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 <[email protected]>
+Date:   Mon Jun 26 00:03:00 2006 +0000
+
+    Side
+
+commit 9a6d4949b6b76956d9d5e26f2791ec2ceff5fdc0
+Author: A U Thor <[email protected]>
+Date:   Mon Jun 26 00:02:00 2006 +0000
+
+    Third
+
+commit 1bde4ae5f36c8d9abe3a0fce0c6aab3c4a12fe44
+Author: A U Thor <[email protected]>
+Date:   Mon Jun 26 00:01:00 2006 +0000
+
+    Second
+    
+    This is the second commit.
+
+commit 444ac553ac7612cc88969031b02b3767fb8a353a
+Author: A U Thor <[email protected]>
+Date:   Mon Jun 26 00:00:00 2006 +0000
+
+    Initial
+$

t/t4013/diff.log_-p_--diff-merges=first-parent_master

@@ -0,0 +1,137 @@
+$ git log -p --diff-merges=first-parent master
+commit 59d314ad6f356dd08601a4cd5e530381da3e3c64
+Merge: 9a6d494 c7a2ab9
+Author: A U Thor <[email protected]>
+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 <[email protected]>
+Date:   Mon Jun 26 00:03:00 2006 +0000
+
+    Side
+
+diff --git a/dir/sub b/dir/sub
+index 35d242b..7289e35 100644
+--- a/dir/sub
++++ b/dir/sub
+@@ -1,2 +1,4 @@
+ A
+ B
++1
++2
+diff --git a/file0 b/file0
+index 01e79c3..f4615da 100644
+--- a/file0
++++ b/file0
+@@ -1,3 +1,6 @@
+ 1
+ 2
+ 3
++A
++B
++C
+diff --git a/file3 b/file3
+new file mode 100644
+index 0000000..7289e35
+--- /dev/null
++++ b/file3
+@@ -0,0 +1,4 @@
++A
++B
++1
++2
+
+commit 9a6d4949b6b76956d9d5e26f2791ec2ceff5fdc0
+Author: A U Thor <[email protected]>
+Date:   Mon Jun 26 00:02:00 2006 +0000
+
+    Third
+
+diff --git a/dir/sub b/dir/sub
+index 8422d40..cead32e 100644
+--- a/dir/sub
++++ b/dir/sub
+@@ -2,3 +2,5 @@ A
+ B
+ C
+ D
++E
++F
+diff --git a/file1 b/file1
+new file mode 100644
+index 0000000..b1e6722
+--- /dev/null
++++ b/file1
+@@ -0,0 +1,3 @@
++A
++B
++C
+
+commit 1bde4ae5f36c8d9abe3a0fce0c6aab3c4a12fe44
+Author: A U Thor <[email protected]>
+Date:   Mon Jun 26 00:01:00 2006 +0000
+
+    Second
+    
+    This is the second commit.
+
+diff --git a/dir/sub b/dir/sub
+index 35d242b..8422d40 100644
+--- a/dir/sub
++++ b/dir/sub
+@@ -1,2 +1,4 @@
+ A
+ B
++C
++D
+diff --git a/file0 b/file0
+index 01e79c3..b414108 100644
+--- a/file0
++++ b/file0
+@@ -1,3 +1,6 @@
+ 1
+ 2
+ 3
++4
++5
++6
+diff --git a/file2 b/file2
+deleted file mode 100644
+index 01e79c3..0000000
+--- a/file2
++++ /dev/null
+@@ -1,3 +0,0 @@
+-1
+-2
+-3
+
+commit 444ac553ac7612cc88969031b02b3767fb8a353a
+Author: A U Thor <[email protected]>
+Date:   Mon Jun 26 00:00:00 2006 +0000
+
+    Initial
+$