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>

Author: dscho

.travis.yml

@@ -39,6 +39,15 @@ env:
 
 matrix:
   include:
+    - env: Linux32
+      os: linux
+      sudo: required
+      services:
+        - docker
+      before_install:
+        - docker pull daald/ubuntu32:xenial
+      before_script:
+      script: ci/run-linux32-build.sh daald/ubuntu32:xenial
     - env: Documentation
       os: linux
       compiler: clang

ci/run-linux32-build.sh

@@ -0,0 +1,31 @@
+#!/bin/sh
+#
+# Build and test Git in a docker container running a 32-bit Ubuntu Linux
+#
+# Usage:
+#   run-linux32-build.sh [container-image]
+#
+
+CONTAINER="${1:-daald/ubuntu32:xenial}"
+
+sudo docker run --interactive --volume "${PWD}:/usr/src/git" "$CONTAINER" \
+    /bin/bash -c 'linux32 --32bit i386 sh -c "
+    : update packages &&
+    apt update >/dev/null &&
+    apt install -y build-essential libcurl4-openssl-dev libssl-dev \
+        libexpat-dev gettext python >/dev/null &&
+
+    : build and test &&
+    cd /usr/src/git &&
+    export DEFAULT_TEST_TARGET='$DEFAULT_TEST_TARGET' &&
+    export GIT_PROVE_OPTS=\"'"$GIT_PROVE_OPTS"'\" &&
+    export GIT_TEST_OPTS=\"'"$GIT_TEST_OPTS"'\" &&
+    export GIT_TEST_CLONE_2GB='$GIT_TEST_CLONE_2GB' &&
+    make --jobs=2 &&
+    make --quiet test || (
+
+    : make test-results readable to non-root user on TravisCI &&
+    test '$TRAVIS' &&
+    find t/test-results/ -type f -exec chmod o+r {} \; &&
+    false )
+"'