Improve upstream parity for level fixtures

- Add native per-level fixtures for complex histories, tags, branch graphs, rebases, resets, restores, stashes, and local remotes.
- Strengthen repository inspection so branch commit counts, remote-tracking branches, and detached tag checkouts reflect real Git state.
- Align Fetch, Pull, Push Branch, Branch At, and Rebase validators/tests with upstream-style repository state.
- Keep the prompt callout and native filesystem tab-completion fixes from the previous batch.

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

@@ -14,7 +14,7 @@ internal fun branchAtLevel(): Level = level(
     hints = listOf("Just like creating a branch, but you have to pass an extra argument."),
     commandSuggestions = listOf("git branch test_branch HEAD~1"),
     setup = { RepoState(initialized = true, commits = listOf(CommitNode("1", "Adding file1"), CommitNode("2", "Updating file1"), CommitNode("3", "Updating file1 again")), branches = mapOf("master" to 3)) },
-    validator = repoPredicate { repo -> "test_branch" in repo.branches },
+    validator = repoPredicate { repo -> repo.branches["test_branch"] == 2 },
     testCases = listOf(
         levelTestCase("branch at previous commit", "git branch test_branch HEAD~1"),
         levelTestCase("branch at caret", "git branch test_branch HEAD^"),