Take all git branches into account for generation of CREDITS file

* Makefile.am: Make CREDITS target depend on all git branches.
diff --git a/Makefile.am b/Makefile.am
index c13b11d..8246d8f 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -110,7 +110,7 @@
 	mv -f $@.new $@
 
 $(srcdir)/CREDITS: $(top_srcdir)/CREDITS.in $(top_srcdir)/.mailmap \
-		   $(srcdir)/Makefile.in
+		   $(srcdir)/Makefile.in $(top_srcdir)/.git/refs/heads/*
 	$(AM_V_GEN)							\
 	{								\
 	  cd $(top_srcdir);						\