Travis: also test on 32-bit Linux

When Git v2.9.1 was released, it had a bug that showed only on Windows
and on 32-bit systems: our assumption that `unsigned long` can hold
64-bit values turned out to be wrong.

This could have been caught earlier if we had a Continuous Testing
set up that includes a build and test run on 32-bit Linux.

Let's do this (and take care of the Windows build later). This patch
asks Travis CI to install a Docker image with 32-bit libraries and then
goes on to build and test Git using this 32-bit setup.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/.travis.yml b/.travis.yml
index 9c63c8c..591cc57 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -39,6 +39,27 @@
 
 matrix:
   include:
+    - env: Linux32
+      os: linux
+      services:
+        - docker
+      before_install:
+        - docker pull daald/ubuntu32:xenial
+      before_script:
+      script:
+        - >
+          docker run
+          --interactive
+          --env DEFAULT_TEST_TARGET
+          --env GIT_PROVE_OPTS
+          --env GIT_TEST_OPTS
+          --env GIT_TEST_CLONE_2GB
+          --volume "${PWD}:/usr/src/git"
+          daald/ubuntu32:xenial
+          /usr/src/git/ci/run-linux32-build.sh $(id -u $USER)
+        # Use the following command to debug the docker build locally:
+        # $ docker run -itv "${PWD}:/usr/src/git" --entrypoint /bin/bash daald/ubuntu32:xenial
+        # root@container:/# /usr/src/git/ci/run-linux32-build.sh
     - env: Documentation
       os: linux
       compiler: clang