additional warning in isGitDir(...)

src/de/uni_passau/fim/seibt/gitwrapper/repo/GitWrapper.java

@@ -184,6 +184,7 @@ public class GitWrapper extends ToolWrapper {
     public boolean isGitDir(File directory) {
 
         if (!directory.isDirectory()) {
+            LOG.warning(() -> String.format("The file '%s' does not exist or is not a directory.", directory));
             return false;
         }