Accept alternate squash solution

- validate the upstream requirement of exactly two commits
- accept soft reset followed by a replacement commit
- reject resetting to one commit without creating the squashed commit
- document squash validation parity

app/src/main/java/solutions/tretter/githugandroid/levels/SquashLevel.kt

@@ -28,11 +28,19 @@ internal fun squashLevel(): Level = level(
         addCommit("Updating README (squash this commit into Adding README)", "README")
         true
     },
-    validator = repoPredicate { repo -> repo.commits.size <= 2 && repo.commits.any { it.message == "Adding README" } },
+    validator = repoPredicate { repo -> repo.commits.size == 2 },
     testCases = listOf(
         levelTestCase(
             "interactive squash",
             "GIT_SEQUENCE_EDITOR=\"sed -i '2,\$s/^pick /squash /'\" GIT_EDITOR=true git rebase -i HEAD~4",
         ),
+        levelTestCase(
+            "soft reset and recommit",
+            "git reset --soft HEAD~4",
+            "git commit -m \"New commit message\"",
+        ),
+    ),
+    negativeTestCases = listOf(
+        levelTestCase("reset without replacement commit", "git reset --soft HEAD~4"),
     ),
 )