contrib/subtree: honor DESTDIR

Teach git-subtree's Makefile to honor DESTDIR.

Signed-off-by: Adam Tkac <atkac@redhat.com>
Signed-off-by: David A. Greene <greened@obbligato.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/subtree/Makefile

@@ -30,12 +30,12 @@ $(GIT_SUBTREE): $(GIT_SUBTREE_SH)
 doc: $(GIT_SUBTREE_DOC)
 
 install: $(GIT_SUBTREE)
-	$(INSTALL) -m 755 $(GIT_SUBTREE) $(libexecdir)
+	$(INSTALL) -m 755 $(GIT_SUBTREE) $(DESTDIR)$(libexecdir)
 
 install-doc: install-man
 
 install-man: $(GIT_SUBTREE_DOC)
-	$(INSTALL) -m 644 $^ $(man1dir)
+	$(INSTALL) -m 644 $^ $(DESTDIR)$(man1dir)
 
 $(GIT_SUBTREE_DOC): $(GIT_SUBTREE_XML)
 	xmlto -m $(MANPAGE_NORMAL_XSL)  man $^