Do not use submodule --single-branch

- It's too new to use, that require git 2.26+

Do not use submodule --single-branch
- It's too new to use, that require git 2.26+
c9e6f335 · Kito Cheng · 65b6e30d · c9e6f335
--- a/Makefile.in
+++ b/Makefile.in
@@ -246,7 +246,7 @@ endif
 $(srcdir)/%/.git:
 	cd $(srcdir) && \
 	flock $(srcdir)/.git/config git submodule init $(dir $@) && \
-	flock $(srcdir)/.git/config git submodule update --single-branch $(dir $@)
+	flock $(srcdir)/.git/config git submodule update $(dir $@)

 #
 # GLIBC