use proper endpoint ... head is implied

Author: mark-i-m

ci/linkcheck.sh

@@ -5,10 +5,10 @@ if [ "$TRAVIS_EVENT_TYPE" = "cron" ] ; then
 
   echo "Doing full link check."
 else
-  CHANGED_FILES=$(git diff-tree --no-commit-id --name-only -r origin/master..HEAD | tr '\n' ' ')
+  CHANGED_FILES=$(git diff-tree --no-commit-id --name-only -r origin/master... | tr '\n' ' ')
   FLAGS="-f $CHANGED_FILES"
 
-  echo "Checking files changed in master..HEAD: $CHANGED_FILES"
+  echo "Checking files changed in origin/master...: $CHANGED_FILES"
 fi
 
 exec mdbook-linkcheck $FLAGS -- $TRAVIS_BUILD_DIR