Fetch kerl with --depth 1

We always want the most recent version and only this.

plugins/ci.mk

@@ -57,7 +57,7 @@ $(foreach otp,$(CI_OTP),$(eval $(call ci_otp_target,$(otp))))
 
 $(KERL):
 	$(verbose) mkdir -p $(ERLANG_MK_TMP)
-	$(gen_verbose) git clone $(KERL_GIT) $(ERLANG_MK_TMP)/kerl
+	$(gen_verbose) git clone --depth 1 $(KERL_GIT) $(ERLANG_MK_TMP)/kerl
 	$(verbose) cd $(ERLANG_MK_TMP)/kerl && git checkout $(KERL_COMMIT)
 	$(verbose) chmod +x $(KERL)