show --first-parent/-m: do not default to --cc

Given that "git show" always shows some diff and does not walk the history
by default, it is natural to expect "git show --first-parent" to show the
difference between the given commit and its first parent.  It also would
be natural, given that "--cc" is the default, "git show -m" to show
pairwise difference from each of the parents.

We however always defaulted to --cc and there was no way to turn it off.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-log.c

@@ -329,6 +329,15 @@ static int show_tree_object(const unsigned char *sha1,
 
 static void show_rev_tweak_rev(struct rev_info *rev, struct setup_revision_opt *opt)
 {
+	if (rev->ignore_merges) {
+		/* There was no "-m" on the command line */
+		rev->ignore_merges = 0;
+		if (!rev->first_parent_only && !rev->combine_merges) {
+			/* No "--first-parent", "-c", nor "--cc" */
+			rev->combine_merges = 1;
+			rev->dense_combined_merges = 1;
+		}
+	}
 	if (!rev->diffopt.output_format)
 		rev->diffopt.output_format = DIFF_FORMAT_PATCH;
 }
@@ -347,10 +356,7 @@ int cmd_show(int argc, const char **argv, const char *prefix)
 
 	init_revisions(&rev, prefix);
 	rev.diff = 1;
-	rev.combine_merges = 1;
-	rev.dense_combined_merges = 1;
 	rev.always_show_header = 1;
-	rev.ignore_merges = 0;
 	rev.no_walk = 1;
 	memset(&opt, 0, sizeof(opt));
 	opt.def = "HEAD";

t/t4013-diff-various.sh

@@ -239,6 +239,8 @@ show --root initial
 show side
 show master
 show -c master
+show -m master
+show --first-parent master
 show --stat side
 show --stat --summary side
 show --patch-with-stat side

t/t4013/diff.show_--first-parent_master

+$ git show --first-parent master
+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
+$

t/t4013/diff.show_-m_master

+$ git show -m master
+commit 59d314ad6f356dd08601a4cd5e530381da3e3c64 (from 9a6d4949b6b76956d9d5e26f2791ec2ceff5fdc0)
+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 59d314ad6f356dd08601a4cd5e530381da3e3c64 (from c7a2ab9e8eac7b117442a607d5a9b3950ae34d5a)
+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 7289e35..992913c 100644
+--- a/dir/sub
++++ b/dir/sub
+@@ -1,4 +1,8 @@
+ A
+ B
++C
++D
++E
++F
+ 1
+ 2
+diff --git a/file0 b/file0
+index f4615da..10a8a9f 100644
+--- a/file0
++++ b/file0
+@@ -1,6 +1,9 @@
+ 1
+ 2
+ 3
++4
++5
++6
+ A
+ B
+ C
+diff --git a/file1 b/file1
+new file mode 100644
+index 0000000..b1e6722
+--- /dev/null
++++ b/file1
+@@ -0,0 +1,3 @@
++A
++B
++C
+diff --git a/file2 b/file2
+deleted file mode 100644
+index 01e79c3..0000000
+--- a/file2
++++ /dev/null
+@@ -1,3 +0,0 @@
+-1
+-2
+-3
+diff --git a/file3 b/file3
+deleted file mode 100644
+index 7289e35..0000000
+--- a/file3
++++ /dev/null
+@@ -1,4 +0,0 @@
+-A
+-B
+-1
+-2
+$