coverage: build coverage-untested-functions by default

Change the 'coverage' target to build coverage-untested-functions by
default, so as to make it more discoverable.
Signed-off-by: NThomas Rast <trast@inf.ethz.ch>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Makefile

@@ -2529,7 +2529,7 @@ check-builtins::
 
 coverage:
 	$(MAKE) coverage-test
-	$(MAKE) coverage-report
+	$(MAKE) coverage-untested-functions
 
 object_dirs := $(sort $(dir $(OBJECTS)))
 coverage-clean-results: