Check for symlink instead of file before install.

scripts/add-code-formatters

@@ -1,4 +1,6 @@
 #!/bin/sh
+
+# Check if file exists with -f. Check if in in the gdk rook directory.
 if [ ! -f ../GDK_ROOT ]; then
   echo "Please run script from gitlab (e.g. gitlab-development-kit/gitlab) root directory."
   exit 1
@@ -6,7 +8,8 @@ fi
 
 PRECOMMIT=$(git rev-parse --git-dir)/hooks/pre-commit
 
-if [ -f $PRECOMMIT ]; then
+# Check if symlink exists with -L. Check if script was already installed.
+if [ -L $PRECOMMIT ]; then
   echo "Pre-commit script already installed."
   exit 1
 fi