Fix wrong glob

By default `**` behaves the same as two `*` side by side, i.e. it only
globs file paths, not directories. `shopt -s globstar` needs to be set
for it to mean a directory. I didn't notice this before now because
`globstar` is set by default in interactive mode, but not otherwise.

ci/check_line_lengths.sh

@@ -10,6 +10,7 @@ if [ "$MAX_LINE_LENGTH" == "" ]; then
 fi
 
 if [ "$1" == "" ]; then
+  shopt -s globstar
   files=( src/**/*.md )
 else
   files=( "$@" )