Makefile: add style build rule

Add the 'style' build rule which will run git-clang-format on the diff
between HEAD and the current worktree.  The result is a diff of
suggested changes.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -2440,6 +2440,10 @@ $(SP_OBJ): %.sp: %.c GIT-CFLAGS FORCE
 .PHONY: sparse $(SP_OBJ)
 sparse: $(SP_OBJ)
 
+.PHONY: style
+style:
+	git clang-format --style file --diff --extensions c,h
+
 check: common-cmds.h
 	@if sparse; \
 	then \