diff: handle revisions pointing to the same sha

Fixes #9604

dvc/repo/diff.py

@@ -156,8 +156,13 @@ def diff(
         for target in old_missing & new_missing:
             raise FileNotFoundError(errno.ENOENT, os.strerror(errno.ENOENT), target)
 
-    old = indexes[a_rev]
-    new = indexes[b_rev]
+    if len(indexes.keys()) == 1:
+        # both a_rev and b_rev point to the same sha, nothing to compare
+        old = None
+        new = None
+    else:
+        old = indexes[a_rev]
+        new = indexes[b_rev]
 
     with ui.status("Calculating diff"):
         return _diff(old, new, with_missing=with_missing)

tests/func/test_diff.py

@@ -21,6 +21,14 @@ def test_no_scm(tmp_dir, dvc):
         dvc.diff()
 
 
+def test_same_rev(tmp_dir, scm, dvc):
+    tmp_dir.dvc_gen("file", "text", commit="add file")
+
+    assert not dvc.diff("HEAD", "HEAD")
+    assert not dvc.diff("HEAD", "master")
+    assert not dvc.diff("master", "HEAD")
+
+
 def test_added(tmp_dir, scm, dvc):
     tmp_dir.dvc_gen("file", "text")