log: add "log.follow" configuration variable

People who work on projects with mostly linear history with frequent
whole file renames may want to always use "git log --follow" when
inspecting the life of the content that live in a single path.

Teach the command to behave as if "--follow" was given from the
command line when log.follow configuration variable is set *and*
there is one (and only one) path on the command line.

Signed-off-by: David Turner <dturner@twopensource.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-log.txt

@@ -184,6 +184,12 @@ log.date::
 	`--date` option.)  Defaults to "default", which means to write
 	dates like `Sat May 8 19:35:34 2010 -0500`.
 
+log.follow::
+	If a single <path> is given to git log, it will act as
+	if the `--follow` option was also used.  This has the same
+	limitations as `--follow`, i.e. it cannot be used to follow
+	multiple files and does not work well on non-linear history.
+
 log.showRoot::
 	If `false`, `git log` and related commands will not treat the
 	initial commit as a big creation event.  Any root commits in

builtin/log.c

@@ -31,6 +31,7 @@ static const char *default_date_mode = NULL;
 
 static int default_abbrev_commit;
 static int default_show_root = 1;
+static int default_follow;
 static int decoration_style;
 static int decoration_given;
 static int use_mailmap_config;
@@ -102,6 +103,8 @@ static void cmd_log_init_defaults(struct rev_info *rev)
 {
 	if (fmt_pretty)
 		get_commit_format(fmt_pretty, rev);
+	if (default_follow)
+		DIFF_OPT_SET(&rev->diffopt, DEFAULT_FOLLOW_RENAMES);
 	rev->verbose_header = 1;
 	DIFF_OPT_SET(&rev->diffopt, RECURSIVE);
 	rev->diffopt.stat_width = -1; /* use full terminal width */
@@ -390,6 +393,10 @@ static int git_log_config(const char *var, const char *value, void *cb)
 		default_show_root = git_config_bool(var, value);
 		return 0;
 	}
+	if (!strcmp(var, "log.follow")) {
+		default_follow = git_config_bool(var, value);
+		return 0;
+	}
 	if (skip_prefix(var, "color.decorate.", &slot_name))
 		return parse_decorate_color_config(var, slot_name, value);
 	if (!strcmp(var, "log.mailmap")) {
@@ -618,6 +625,14 @@ int cmd_log_reflog(int argc, const char **argv, const char *prefix)
 	return cmd_log_walk(&rev);
 }
 
+static void default_follow_tweak(struct rev_info *rev,
+				 struct setup_revision_opt *opt)
+{
+	if (DIFF_OPT_TST(&rev->diffopt, DEFAULT_FOLLOW_RENAMES) &&
+	    rev->prune_data.nr == 1)
+		DIFF_OPT_SET(&rev->diffopt, FOLLOW_RENAMES);
+}
+
 int cmd_log(int argc, const char **argv, const char *prefix)
 {
 	struct rev_info rev;
@@ -631,6 +646,7 @@ int cmd_log(int argc, const char **argv, const char *prefix)
 	memset(&opt, 0, sizeof(opt));
 	opt.def = "HEAD";
 	opt.revarg_opt = REVARG_COMMITTISH;
+	opt.tweak = default_follow_tweak;
 	cmd_log_init(argc, argv, prefix, &rev, &opt);
 	return cmd_log_walk(&rev);
 }

diff.c

@@ -3815,9 +3815,10 @@ int diff_opt_parse(struct diff_options *options, const char **av, int ac)
 		DIFF_OPT_SET(options, FIND_COPIES_HARDER);
 	else if (!strcmp(arg, "--follow"))
 		DIFF_OPT_SET(options, FOLLOW_RENAMES);
-	else if (!strcmp(arg, "--no-follow"))
+	else if (!strcmp(arg, "--no-follow")) {
 		DIFF_OPT_CLR(options, FOLLOW_RENAMES);
-	else if (!strcmp(arg, "--color"))
+		DIFF_OPT_CLR(options, DEFAULT_FOLLOW_RENAMES);
+	} else if (!strcmp(arg, "--color"))
 		options->use_color = 1;
 	else if (skip_prefix(arg, "--color=", &arg)) {
 		int value = git_config_colorbool(NULL, arg);

diff.h

@@ -91,6 +91,7 @@ typedef struct strbuf *(*diff_prefix_fn_t)(struct diff_options *opt, void *data)
 #define DIFF_OPT_DIRSTAT_BY_LINE     (1 << 28)
 #define DIFF_OPT_FUNCCONTEXT         (1 << 29)
 #define DIFF_OPT_PICKAXE_IGNORE_CASE (1 << 30)
+#define DIFF_OPT_DEFAULT_FOLLOW_RENAMES (1 << 31)
 
 #define DIFF_OPT_TST(opts, flag)    ((opts)->flags & DIFF_OPT_##flag)
 #define DIFF_OPT_TOUCHED(opts, flag)    ((opts)->touched_flags & DIFF_OPT_##flag)

t/t4202-log.sh

@@ -146,7 +146,30 @@ test_expect_success 'git log --follow' '
 	actual=$(git log --follow --pretty="format:%s" ichi) &&
 	expect=$(echo third ; echo second ; echo initial) &&
 	verbose test "$actual" = "$expect"
+'
+
+test_expect_success 'git config log.follow works like --follow' '
+	test_config log.follow true &&
+	actual=$(git log --pretty="format:%s" ichi) &&
+	expect=$(echo third ; echo second ; echo initial) &&
+	verbose test "$actual" = "$expect"
+'
 
+test_expect_success 'git config log.follow does not die with multiple paths' '
+	test_config log.follow true &&
+	git log --pretty="format:%s" ichi ein
+'
+
+test_expect_success 'git config log.follow does not die with no paths' '
+	test_config log.follow true &&
+	git log --
+'
+
+test_expect_success 'git config log.follow is overridden by --no-follow' '
+	test_config log.follow true &&
+	actual=$(git log --no-follow --pretty="format:%s" ichi) &&
+	expect="third" &&
+	verbose test "$actual" = "$expect"
 '
 
 cat > expect << EOF