log: add log.mailmap configuration option

Teach "log.mailmap" configuration variable to turn "--use-mailmap"
option on to "git log", "git show" and "git whatchanged".

The "--no-use-mailmap" option from the command line can countermand
the setting.

Signed-off-by: Antoine Pelisse <apelisse@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1509,6 +1509,10 @@ log.showroot::
 	Tools like linkgit:git-log[1] or linkgit:git-whatchanged[1], which
 	normally hide the root commit will now show it. True by default.
 
+log.mailmap::
+	If true, makes linkgit:git-log[1], linkgit:git-show[1], and
+	linkgit:git-whatchanged[1] assume `--use-mailmap`.
+
 mailmap.file::
 	The location of an augmenting mailmap file. The default
 	mailmap, located in the root of the repository, is loaded

builtin/log.c

@@ -31,6 +31,7 @@ static int default_abbrev_commit;
 static int default_show_root = 1;
 static int decoration_style;
 static int decoration_given;
+static int use_mailmap_config;
 static const char *fmt_patch_subject_prefix = "PATCH";
 static const char *fmt_pretty;
 
@@ -106,6 +107,7 @@ static void cmd_log_init_finish(int argc, const char **argv, const char *prefix,
 		OPT_END()
 	};
 
+	mailmap = use_mailmap_config;
 	argc = parse_options(argc, argv, prefix,
 			     builtin_log_options, builtin_log_usage,
 			     PARSE_OPT_KEEP_ARGV0 | PARSE_OPT_KEEP_UNKNOWN |
@@ -358,6 +360,11 @@ static int git_log_config(const char *var, const char *value, void *cb)
 	}
 	if (!prefixcmp(var, "color.decorate."))
 		return parse_decorate_color_config(var, 15, value);
+	if (!strcmp(var, "log.mailmap")) {
+		use_mailmap_config = git_config_bool(var, value);
+		return 0;
+	}
+
 	if (grep_config(var, value, cb) < 0)
 		return -1;
 	return git_diff_ui_config(var, value, cb);

t/t4203-mailmap.sh

@@ -254,6 +254,21 @@ test_expect_success 'Log output with --use-mailmap' '
 	test_cmp expect actual
 '
 
+cat >expect <<\EOF
+Author: CTO <cto@company.xx>
+Author: Santa Claus <santa.claus@northpole.xx>
+Author: Santa Claus <santa.claus@northpole.xx>
+Author: Other Author <other@author.xx>
+Author: Other Author <other@author.xx>
+Author: Some Dude <some@dude.xx>
+Author: A U Thor <author@example.com>
+EOF
+
+test_expect_success 'Log output with log.mailmap' '
+	git -c log.mailmap=True log | grep Author >actual &&
+	test_cmp expect actual
+'
+
 cat >expect <<\EOF
 Author: Santa Claus <santa.claus@northpole.xx>
 Author: Santa Claus <santa.claus@northpole.xx>
@@ -263,6 +278,15 @@ test_expect_success 'Grep author with --use-mailmap' '
 	git log --use-mailmap --author Santa | grep Author >actual &&
 	test_cmp expect actual
 '
+cat >expect <<\EOF
+Author: Santa Claus <santa.claus@northpole.xx>
+Author: Santa Claus <santa.claus@northpole.xx>
+EOF
+
+test_expect_success 'Grep author with log.mailmap' '
+	git -c log.mailmap=True log --author Santa | grep Author >actual &&
+	test_cmp expect actual
+'
 
 >expect