TST: travis: Adjust upstream Git build for Xenial

Travis recently switched to using Xenial by default, and the Xenial
image, unlike the Trusty one, doesn't have third-party
apt-repositories [0].  Selectively enable ppa:git-core/ppa in the
upstream Git build because we rely on that repository to get the
latest Git.

Note that we install git through "addons: apt: packages:" rather than
our tools/ci/install-latest-git.sh script.  This has the downside that
Git is installed unnecessarily, both for builds that have _DL_CRON set
and for builds where the bundled Git matches the latest upstream one.
We do it because otherwise the build exits (with a status of 0) right
after the installation happens [1].  The same happens if we use "dist:
trusty", which should have the third-party repositories enabled [2].

[0]: https://docs.travis-ci.com/user/reference/xenial/#third-party-apt-repositories-removed
[1]: https://travis-ci.org/datalad/datalad/jobs/550376442
[2]: https://travis-ci.org/datalad/datalad/builds/550835880

Re: datalad#3476

.travis.yml

@@ -125,6 +125,12 @@ matrix:
     - _DL_CRON=1
   # Run with latest git rather than the bundled one.
   - python: 3.5
+    addons:
+      apt:
+        sources:
+          - sourceline: 'ppa:git-core/ppa'
+        packages:
+          - git
     env:
     - DATALAD_USE_DEFAULT_GIT=1
     - _DL_UPSTREAM_GIT=1
@@ -262,8 +268,8 @@ before_install:
   - if [ ! -z "${_DL_UPSTREAM_GITANNEX:-}" ]; then sudo tools/ci/install-annex-snapshot.sh; sudo ln -s `find /usr/local/lib/git-annex.linux -maxdepth 1 -type f -perm /+x` /usr/local/bin/; else sudo eatmydata apt-get install git-annex-standalone ; fi
   # Install optionally -devel version of annex, and if goes wrong (we have most recent), exit right away
   - if [ ! -z "${_DL_DEVEL_ANNEX:-}" ]; then tools/ci/prep-travis-devel-annex.sh || exit 0; fi
-  # Optionally install the latest Git.  Exit code 100 indicates that bundled is same as the latest.
-  - if [ ! -z "${_DL_UPSTREAM_GIT:-}" ]; then sudo tools/ci/install-latest-git.sh || [ $? -eq 100 ] && exit 0; fi
+  # Exit code 100 indicates that bundled is same as the latest.
+  - if [ ! -z "${_DL_UPSTREAM_GIT:-}" ]; then sudo tools/ci/check-latest-git.sh || [ $? -eq 100 ] && exit 0; fi
 
 install:
   # Install standalone build of git-annex for the recent enough version

tools/ci/install-latest-git.sh → tools/ci/check-latest-git.sh

@@ -2,7 +2,9 @@
 
 set -e -u
 
-# NOTE: Travis already has ppa:git-core/ppa set up.
+# NOTE: In .travis.yml, we've already set up ppa:git-core/ppa and installed the
+# latest git. Ideally, we'd do the install here, but doing so causes the build
+# exit right after for some reason.
 
                      # e.g., Candidate: 1:2.19.1-1~ppa0~ubuntu14.04.1
 latest_git_version=$(apt-cache policy git | awk -F'[:-]' '/Candidate/{print $3;}')
@@ -19,6 +21,4 @@ elif [ -z "$latest_git_version" ]; then
 elif [ "$latest_git_version" = "$bundled_git_version" ]; then
     echo "I: latest git version $latest_git_version is same as bundled $bundled_git_version"
     exit 100
-else
-    sudo apt-get install -y git
 fi