src/diff.rs: add a diff subcommand to compute the difference between two commits

src/diff.rs

+use crate::checkout;
+use crate::commit;
+
+pub fn compute_diff(commit1: &str, commit2: &Option<String>) {
+    let (borg_path, _) = commit::check_path();
+    match commit2 {
+        Some(c2) => {
+            checkout::is_diff(commit1, c2, &borg_path);
+        }
+        None => {
+            // Create an archive with the content of the current commit
+            let tmp_name = format!("{}-tmp", commit1);
+            checkout::silent_commit(String::from("none"), tmp_name.clone(), false);
+            checkout::is_diff(&commit1, &tmp_name, &borg_path);
+            commit::delete_commit(&tmp_name, &borg_path);
+        }
+    }
+}