chore(travis): custom merge pubspec.lock for g3v1x rebases

pubspec.lock changes will never really apply cleanly to g3v1x.  What we
wnat to capture, is whether Travis tests will pass if we apply the
pubspec.yaml changes to the g3v1x branch and then regenerate the
pubspec.lock file.  This commit inserts a custom merge driver for git to
ignore changes to the pubspec.lock file during merging to achieve this.

Author: chirayuk

scripts/travis/setup.sh

@@ -9,6 +9,15 @@ if [ "$USE_G3" = "YES" ]; then
   git config --global user.name "AngularDart on Travis"
   git remote add upstream https://github.com/angular/angular.dart.git
   git fetch upstream
+  # Set up a custom merge driver so that top level pubspec.lock conflicts for
+  # do not fail the rebase.
+  echo $'pubspec.lock merge=merge_pubspec_lock\n' >> .git/info/attributes
+  cat >> .git/config <<"EOF"
+[merge "merge_pubspec_lock"]
+  name = "Auto merge pubspec.lock conflicts by ignoring changes"
+  driver = cp %O %A
+  recursive = text
+EOF
   git rebase --onto upstream/g3v1x upstream/g3v1x-master
 fi