Fix "arc:this" and "arc:amended" base rules after D4838

Summary:
After D4383, we escape the base commit when constructing a command like this:

  hg log --rev (base::. - base)

However, if the base commit is a revset like ".^", we now escape it and Mercurial looks for a commit named ".^" (a valid mercurial branch name) instead.

Fix this by returning nodes for these rules instead of revsets. The "arc:this" rule is automatically used in some operations, like "arc amend", so users can hit this during normal workflows, not just with weird `--base` rules.

Test Plan: Ran "arc amend" in a Mercurial repository, didn't fatal out.

Reviewers: DurhamGoode, sid0

Reviewed By: DurhamGoode

CC: tido, aran

Differential Revision: https://secure.phabricator.com/D4949

src/repository/api/ArcanistMercurialAPI.php

@@ -672,6 +672,11 @@ final class ArcanistMercurialAPI extends ArcanistRepositoryAPI {
   public function resolveBaseCommitRule($rule, $source) {
     list($type, $name) = explode(':', $rule, 2);
 
+    // NOTE: This function MUST return node hashes or symbolic commits (like
+    // branch names or the word "tip"), not revsets. This includes ".^" and
+    // similar, which a revset, not a symbolic commit identifier. If you return
+    // a revset it will be escaped later and looked up literally.
+
     switch ($type) {
       case 'hg':
         $matches = null;
@@ -728,7 +733,7 @@ final class ArcanistMercurialAPI extends ArcanistRepositoryAPI {
                 "configuration.");
               // NOTE: This should be safe because Mercurial doesn't support
               // amend until 2.2.
-              return '.^';
+              return $this->getCanonicalRevisionName('.^');
             }
             break;
           case 'bookmark':
@@ -754,7 +759,7 @@ final class ArcanistMercurialAPI extends ArcanistRepositoryAPI {
             $this->setBaseCommitExplanation(
               "you specified '{$rule}' in your {$source} 'base' ".
               "configuration.");
-            return '.^';
+            return $this->getCanonicalRevisionName('.^');
         }
         break;
       default: