contrib/subtree: make the manual directory if needed

Before install git-subtree documentation, make sure the manpage
directory exists.

Signed-off-by: Jesper L. Nielsen <lyager@gmail.com>
Signed-off-by: David A. Greene <greened@obbligato.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/subtree/Makefile

@@ -35,6 +35,7 @@ install: $(GIT_SUBTREE)
 install-doc: install-man
 
 install-man: $(GIT_SUBTREE_DOC)
+	$(INSTALL) -d -m 755 $(DESTDIR)$(man1dir)
 	$(INSTALL) -m 644 $^ $(DESTDIR)$(man1dir)
 
 $(GIT_SUBTREE_DOC): $(GIT_SUBTREE_XML)