I reported this a while ago but evidently forgot to push the patch
upstream. Without this I wasn't seeing the dirty tree state submitted
to the docker build which is very confusing.

Reference:
  Subject: [PATCH 1/6] tests/docker/Makefile.include: fix diff-index call
  Date: Fri, 28 Oct 2016 17:33:34 +0100
  Message-Id: <20161028163339.31096-2-alex.bennee@linaro.org>

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
---
 tests/docker/Makefile.include | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/docker/Makefile.include b/tests/docker/Makefile.include
index d7dafdbd27..ff580edfad 100644
--- a/tests/docker/Makefile.include
+++ b/tests/docker/Makefile.include
@@ -20,7 +20,7 @@ IMAGES ?= %
 # Make archive from git repo $1 to tar.gz $2
 make-archive-maybe = $(if $(wildcard $1/*), \
 	$(call quiet-command, \
-		(cd $1; if git diff-index --quiet HEAD -- &>/dev/null; then \
+		(cd $1; if git diff-index --quiet HEAD -- 2>/dev/null; then \
 			git archive -1 HEAD --format=tar.gz; \
 		else \
 			git archive -1 $$(git stash create) --format=tar.gz; \
-- 
2.13.0