Replaced previous version of spell check with version that uses output

of git diff.

doc.zih.tu-dresden.de/util/check-spelling-changes.sh

@@ -19,18 +19,33 @@ fi
 any_fails=false
 
 source_hash=`git merge-base HEAD "$branch"`
-files=$(git diff --name-only "$source_hash")
-for f in $files; do
-    if [ "${f: -3}" == ".md" ]; then
-        previous_count=`git show "$source_hash:$f" | getNumberOfAspellOutputLines`
-        current_count=`cat "$f" | getNumberOfAspellOutputLines`
+#Remove everything except lines beginning with --- or +++
+files=`git diff $source_hash | sed -n 's/^[-+]\{3,3\} //p'`
+#Assume that we have pairs of lines (starting with --- and +++).
+while read oldfile; do
+    read newfile
+    if [ "${newfile: -3}" == ".md" ]; then
+        if [ "$oldfile" == "/dev/null" ]; then
+            #Added files should not introduce new spelling mistakes
+            previous_count=0
+        else
+            previous_count=`git show "$source_hash:${oldfile:2}" | getNumberOfAspellOutputLines`
+        fi
+        if [ "$newfile" == "/dev/null" ]; then
+            #Deleted files do not contain any spelling mistakes
+            current_count=0
+        else
+            #Remove the prefix "b/"
+            newfile=${newfile:2}
+            current_count=`cat "$newfile" | getNumberOfAspellOutputLines`
+        fi
         if [ $current_count -gt $previous_count ]; then
-            echo "-- File $f"
+            echo "-- File $newfile"
             echo "Change increases spelling mistake count (from $previous_count to $current_count)"
             any_fails=true
         fi
     fi
-done
+done <<< "$files"
 
 if [ "$any_fails" == true ]; then
     exit 1