fix commit ranges, attempt rust-lang#2

Author: mark-i-m

ci/linkcheck.sh

@@ -1,14 +1,20 @@
 #!/usr/bin/env bash
 
-if [ "$TRAVIS_EVENT_TYPE" = "cron" ] ; then
+if [ "$TRAVIS_EVENT_TYPE" = "cron" ] ; then # running in cron job
   FLAGS=""
 
   echo "Doing full link check."
-else
-  CHANGED_FILES=$(git diff-tree --no-commit-id --name-only -r origin/master... | tr '\n' ' ')
+elif [ "$CI" = "true" ] ; then # running in PR CI build
+  CHANGED_FILES=$(git diff-tree --no-commit-id --name-only -r $TRAVIS_COMMIT_RANGE | tr '\n' ' ')
   FLAGS="-f $CHANGED_FILES"
 
-  echo "Checking files changed in origin/master...: $CHANGED_FILES"
+  echo "Checking files changed in $TRAVIS_COMMIT_RANGE: $CHANGED_FILES"
+else # running locally
+  COMMIT_RANGE=master..
+  CHANGED_FILES=$(git diff-tree --no-commit-id --name-only -r $COMMIT_RANGE | tr '\n' ' ')
+  FLAGS="-f $CHANGED_FILES"
+
+  echo "Checking files changed in $COMMIT_RANGE: $CHANGED_FILES"
 fi
 
 exec mdbook-linkcheck $FLAGS -- $TRAVIS_BUILD_DIR