status: introduce status.branch to enable --branch by default

Some people often run 'git status -b'.
The config variable status.branch allows to set it by default.

Signed-off-by: Jorge Juan Garcia Garcia <Jorge-Juan.Garcia-Garcia@ensimag.imag.fr>
Signed-off-by: Mathieu Lienard--Mayor <Mathieu.Lienard--Mayor@ensimag.imag.fr>
Reviewed-by: Matthieu Moy <Matthieu.Moy@grenoble-inp.fr>
Signed-off-by: Matthieu Moy <Matthieu.Moy@grenoble-inp.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/commit.c b/builtin/commit.c
index b2f41de..d6c8e20 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1117,6 +1117,10 @@
 			status_format = STATUS_FORMAT_NONE;
 		return 0;
 	}
+	if (!strcmp(k, "status.branch")) {
+		s->show_branch = git_config_bool(k, v);
+		return 0;
+	}
 	if (!strcmp(k, "status.color") || !strcmp(k, "color.status")) {
 		s->use_color = git_config_colorbool(k, v);
 		return 0;