Makefile: disable default implicit rules

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>