Added method to check if a Reference contains a certain file.

src/de/uni_passau/fim/gitwrapper/Reference.java

 package de.uni_passau.fim.gitwrapper;
 
+import java.nio.file.Path;
 import java.util.Arrays;
 import java.util.List;
 import java.util.Objects;
@@ -126,6 +127,18 @@ public abstract class Reference {
         return revList.map(toParentsList);
     }
 
+    /**
+     * Checks if this {@link Commit} contains the specified file.
+     *
+     * @param file The file to check the {@link Commit} for existence.
+     * @return {@code True} if the {@link Commit} contains the file, {@code False} otherwise.
+     */
+    public boolean hasFile(Path file) {
+        Optional<ProcessExecutor.ExecRes> result = git.exec(repo.getDir(), "cat-file",
+                "-e", id + ":" + file);
+        return result.get().succeeded();
+    }
+
     @Override
     public final boolean equals(Object o) {
         if (this == o) {