contrib/subtree: Fix make install target

If the libexec directory doesn't exist, git-subtree gets installed as
$prefix/share/libexec/git-core file. This patch creates the directory
before installing git-subtree file into it.

Signed-off-by: Michal Sojka <sojkam1@fel.cvut.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/subtree/Makefile

@@ -30,6 +30,7 @@ $(GIT_SUBTREE): $(GIT_SUBTREE_SH)
 doc: $(GIT_SUBTREE_DOC)
 
 install: $(GIT_SUBTREE)
+	$(INSTALL) -d -m 755 $(DESTDIR)$(libexecdir)
 	$(INSTALL) -m 755 $(GIT_SUBTREE) $(DESTDIR)$(libexecdir)
 
 install-doc: install-man