Fix existence test in "reconcile.php"

Summary:
"rev-parse --verify" is actually a terrible test, it only survived my test cases
because I put garbage into the tables with RAND() or similar, not
properly-formatted garbage.

Use "cat-file -t" instead to ensure we perform an object-existence test.

Test Plan: Ran "reconcile.php P" locally, ran "cat-file -t" and "rev-parse
--verify" on properly-formatted but invalid hashes like
"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", worked with @rguerin to resolve
commit issues.

Reviewers: btrahan, rguerin

Reviewed By: rguerin

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1590

scripts/repository/reconcile.php

@@ -88,8 +88,11 @@ try {
 echo "Verifying commits (this may take some time if the repository is large)";
 $futures = array();
 foreach ($all_commits as $id => $commit) {
+  // NOTE: We use "cat-file -t", not "rev-parse --verify", because
+  // "rev-parse --verify" does not verify that the object actually exists, only
+  // that the name is properly formatted.
   $futures[$id] = $repository->getLocalCommandFuture(
-    'rev-parse --verify %s',
+    'cat-file -t %s',
     $commit->getCommitIdentifier());
 }