Support "immutable_history" doctrine in arc

Summary: When a project uses a conservative history mutability doctrine, never
try to amend history.

Test Plan: Ran "arc amend" in an "immutable_history" working copy.

Reviewers: fratrik, Makinde, aran, jungejason, tuomaspelkonen

Reviewed By: Makinde

CC: aran, Makinde, epriestley

Differential Revision: 862

src/workflow/amend/ArcanistAmendWorkflow.php

@@ -65,6 +65,14 @@ EOTEXT
   }
 
   public function run() {
+    if ($this->isHistoryImmutable()) {
+      throw new ArcanistUsageException(
+        "This project is marked as adhering to a conservative history ".
+        "mutability doctrine (having an immutable local history), which ".
+        "precludes amending commit messages. You can use 'arc merge' to ".
+        "merge feature branches instead.");
+    }
+
     $repository_api = $this->getRepositoryAPI();
     if (!($repository_api instanceof ArcanistGitAPI)) {
       throw new ArcanistUsageException(

src/workflow/base/ArcanistBaseWorkflow.php

@@ -913,4 +913,9 @@ class ArcanistBaseWorkflow {
     file_put_contents('php://stderr', $msg);
   }
 
+  protected function isHistoryImmutable() {
+    $working_copy = $this->getWorkingCopy();
+    return ($working_copy->getConfig('immutable_history') === true);
+  }
+
 }

src/workflow/diff/ArcanistDiffWorkflow.php

@@ -549,8 +549,10 @@ EOTEXT
     }
 
     if ($repository_api instanceof ArcanistMercurialAPI) {
-      // TODO: This is unlikely to be correct since it excludes using local
-      // branching in Mercurial.
+      return true;
+    }
+
+    if ($this->isHistoryImmutable()) {
       return true;
     }
 
@@ -1110,7 +1112,10 @@ EOTEXT
       }
       $lint_workflow = $this->buildChildWorkflow('lint', $argv);
 
+      if (!$this->isHistoryImmutable()) {
+        // TODO: We should offer to create a checkpoint commit.
         $lint_workflow->setShouldAmendChanges(true);
+      }
 
       $lint_result = $lint_workflow->run();