contrib/subtree/Makefile: clean up rule for "clean"

git:Documentation/Makefile and others establish "RM ?= rm -f" as a
convention for rm calls in clean rules, hence follow this convention
instead of simply forcing clean to use rm.

subproj and mainline no longer need to be removed in clean, as they are
no longer created in git:contrib/subtree by "make test". Hence, remove
the rm call for those folders.

Other makefiles don't remove "*~" files, remove the rm call to prevent
unexpected behaviour in the future. Similarly, clean doesn't remove the
installable file, so rectify this.

Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: James Denholm <nod.helm@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/subtree/Makefile

@@ -12,7 +12,8 @@ man1dir ?= $(mandir)/man1
 -include ../../GIT-VERSION-FILE
 
 # this should be set to a 'standard' bsd-type install program
-INSTALL ?= install
+INSTALL  ?= install
+RM       ?= rm -f
 
 ASCIIDOC = asciidoc
 XMLTO    = xmlto
@@ -60,7 +61,7 @@ test:
 	$(MAKE) -C t/ test
 
 clean:
-	rm -f *~ *.xml *.html *.1
-	rm -rf subproj mainline
+	$(RM) $(GIT_SUBTREE)
+	$(RM) *.xml *.html *.1
 
 .PHONY: FORCE