git-status: create config for ahead/behind calculation

index 1c9e8e2228..71305073ad 100644
@@ -1078,9 +1078,11 @@ static const char *read_commit_message(const char *name)
 static struct status_deferred_config {
 	enum wt_status_format status_format;
 	int show_branch;
+	enum ahead_behind_flags ahead_behind;
 } status_deferred_config = {
 	STATUS_FORMAT_UNSPECIFIED,
-	-1 /* unspecified */
+	-1, /* unspecified */
+	AHEAD_BEHIND_UNSPECIFIED,
 };
 
 static void finalize_deferred_config(struct wt_status *s)

@@ -1107,6 +1109,15 @@ static void finalize_deferred_config(struct wt_status *s)
 	if (s->show_branch < 0)
 		s->show_branch = 0;
 
+	/*
+	 * If the user did not give a "--[no]-ahead-behind" command
+	 * line argument, then we inherit the a/b config setting.
+	 * If is not set, then we inherit _FULL for backwards
+	 * compatibility.
+	 */
+	if (s->ahead_behind_flags == AHEAD_BEHIND_UNSPECIFIED)
+		s->ahead_behind_flags = status_deferred_config.ahead_behind;
+
 	if (s->ahead_behind_flags == AHEAD_BEHIND_UNSPECIFIED)
 		s->ahead_behind_flags = AHEAD_BEHIND_FULL;
 }

@@ -1246,6 +1257,10 @@ static int git_status_config(const char *k, const char *v, void *cb)
 		status_deferred_config.show_branch = git_config_bool(k, v);
 		return 0;
 	}
+	if (!strcmp(k, "status.aheadbehind")) {
+		status_deferred_config.ahead_behind = git_config_bool(k, v);
+		return 0;
+	}
 	if (!strcmp(k, "status.showstash")) {
 		s->show_stash = git_config_bool(k, v);
 		return 0;

index ce5f374ecd..54f8dea30c 100644
@@ -12,6 +12,7 @@ int advice_push_needs_force = 1;
 int advice_push_unqualified_ref_name = 1;
 int advice_status_hints = 1;
 int advice_status_u_option = 1;
+int advice_status_ahead_behind_warning = 1;
 int advice_commit_before_merge = 1;
 int advice_reset_quiet_warning = 1;
 int advice_resolve_conflict = 1;

@@ -68,6 +69,7 @@ static struct {
 	{ "pushUnqualifiedRefName", &advice_push_unqualified_ref_name },
 	{ "statusHints", &advice_status_hints },
 	{ "statusUoption", &advice_status_u_option },
+	{ "statusAheadBehindWarning", &advice_status_ahead_behind_warning },
 	{ "commitBeforeMerge", &advice_commit_before_merge },
 	{ "resetQuiet", &advice_reset_quiet_warning },
 	{ "resolveConflict", &advice_resolve_conflict },

index e50f02cdfe..c86de9b9b8 100644
@@ -12,6 +12,7 @@ extern int advice_push_needs_force;
 extern int advice_push_unqualified_ref_name;
 extern int advice_status_hints;
 extern int advice_status_u_option;
+extern int advice_status_ahead_behind_warning;
 extern int advice_commit_before_merge;
 extern int advice_reset_quiet_warning;
 extern int advice_resolve_conflict;

index d2a1bec226..c94d43879a 100644
@@ -19,6 +19,8 @@
 #include "lockfile.h"
 #include "sequencer.h"
 
+#define AB_DELAY_WARNING_IN_MS (2 * 1000)
+
 static const char cut_line[] =
 "------------------------ >8 ------------------------\n";
 

@@ -1085,14 +1087,29 @@ static void wt_longstatus_print_tracking(struct wt_status *s)
 	struct branch *branch;
 	char comment_line_string[3];
 	int i;
+	uint64_t t_begin = 0;
 
 	assert(s->branch && !s->is_initial);
 	if (!skip_prefix(s->branch, "refs/heads/", &branch_name))
 		return;
 	branch = branch_get(branch_name);
+
+	t_begin = getnanotime();
+
 	if (!format_tracking_info(branch, &sb, s->ahead_behind_flags))
 		return;
 
+	if (advice_status_ahead_behind_warning &&
+	    s->ahead_behind_flags == AHEAD_BEHIND_FULL) {
+		uint64_t t_delta_in_ms = (getnanotime() - t_begin) / 1000000;
+		if (t_delta_in_ms > AB_DELAY_WARNING_IN_MS) {
+			strbuf_addf(&sb, _("\n"
+					   "It took %.2f seconds to compute the branch ahead/behind values.\n"
+					   "You can use '--no-ahead-behind' to avoid this.\n"),
+				    t_delta_in_ms / 1000.0);
+		}
+	}
+
 	i = 0;
 	if (s->display_comment_prefix) {
 		comment_line_string[i++] = comment_line_char;

index 71305073ad..79cb238d87 100644
@@ -1111,11 +1111,13 @@ static void finalize_deferred_config(struct wt_status *s)
 
 	/*
 	 * If the user did not give a "--[no]-ahead-behind" command
-	 * line argument, then we inherit the a/b config setting.
-	 * If is not set, then we inherit _FULL for backwards
-	 * compatibility.
+	 * line argument *AND* we will print in a human-readable format
+	 * (short, long etc.) then we inherit from the status.aheadbehind
+	 * config setting.  In all other cases (and porcelain V[12] formats
+	 * in particular), we inherit _FULL for backwards compatibility.
 	 */
-	if (s->ahead_behind_flags == AHEAD_BEHIND_UNSPECIFIED)
+	if (use_deferred_config &&
+	    s->ahead_behind_flags == AHEAD_BEHIND_UNSPECIFIED)
 		s->ahead_behind_flags = status_deferred_config.ahead_behind;
 
 	if (s->ahead_behind_flags == AHEAD_BEHIND_UNSPECIFIED)