Makefile: disable default implicit rules

The git makefile never uses any default implicit rules. Unfortunately, if a prerequisite for one of the intended rules is missing, a default rule can be used in its place: $ make var.s CC var.s $ rm var.c $ make var.o as -o var.o var.s Avoiding the default rules avoids this hard-to-debug behavior. It also should speed things up a little in the normal case. Future patches may restrict the scope of the %.o: %.c pattern. This patch would then ensure that for targets not listed, we do not fall back to the default rule. Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>