Skip CICD build on markdown or doc changes.

consider-early-travis-exit.sh

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+# Exits with status 0 if it can be determined that the
+# current PR should not trigger all travis checks.
+#
+# This could be done with a "git ...|grep -vqE" oneliner
+# but as travis triggering is refined it's useful to check
+# travis logs to see how branch files were considered.
+function consider-early-travis-exit {
+  if [ "$TRAVIS_PULL_REQUEST" == "false" ]; then
+    echo "Not a travis pull request."
+    return
+  fi
+  if [ -z "$TRAVIS_BRANCH" ]; then
+    echo "Unknown travis branch."
+    return
+  fi
+  echo "TRAVIS_BRANCH=$TRAVIS_BRANCH"
+  local branchFiles=$(git diff --name-only FETCH_HEAD...$TRAVIS_BRANCH)
+  local invisibles=0
+  local triggers=0
+  echo "Branch Files (X==invisible to travis):"
+  echo "---"
+  for fn in $branchFiles; do
+    if [[ "$fn" =~ (\.md$)|(^docs/) ]]; then
+      echo "  X  $fn"
+      let invisibles+=1
+    else
+      echo "     $fn"
+      let triggers+=1
+    fi
+  done
+  echo "---"
+  printf >&2 "%6d files invisible to travis.\n" $invisibles
+  printf >&2 "%6d files trigger travis.\n" $triggers
+  if [ $triggers -eq 0 ]; then
+    echo "Exiting travis early."
+    # see https://github.com/travis-ci/travis-build/blob/master/lib/travis/build/templates/header.sh
+    travis_terminate 0
+  fi
+}
+consider-early-travis-exit
+unset -f consider-early-travis-exit