Provide basic support for Subversion revprops

Summary:
Ref T11208. See that task for a more detailed description of revprops.

This allows revprop changes in a hosted Subversion repository if the repository has the "allow dangerous changes" flag set.

In the future, we could expand this into real Herald support, but the only use case we have for now is letting `svnsync` work.

Test Plan:
Edited revprops with `svn propset --revprop -r 2 propkey propvalue repositoryuri`:

  - Tried before patch, got a "configure a commit hook" error.
  - Tried after patch, got a "dangerous change" error.
  - Allowed dangerous changes.
  - Did a revprop edit.
  - Prevented dangerous changes.
  - Got an error again.
  - Made a normal commit to an SVN repository.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11208

Differential Revision: https://secure.phabricator.com/D16174

scripts/repository/commit_hook.php

@@ -54,10 +54,62 @@ if (!$repository->isHosted()) {
 
 $engine->setRepository($repository);
 
+$args = new PhutilArgumentParser($argv);
+$args->parsePartial(
+  array(
+    array(
+      'name' => 'hook-mode',
+      'param' => 'mode',
+      'help' => pht('Hook execution mode.'),
+    ),
+  ));
+
+$argv = array_merge(
+  array($argv[0]),
+  $args->getUnconsumedArgumentVector());
 
 // Figure out which user is writing the commit.
+$hook_mode = $args->getArg('hook-mode');
+if ($hook_mode !== null) {
+  $known_modes = array(
+    'svn-revprop' => true,
+  );
 
-if ($repository->isGit() || $repository->isHg()) {
+  if (empty($known_modes[$hook_mode])) {
+    throw new Exception(
+      pht(
+        'Invalid Hook Mode: This hook was invoked in "%s" mode, but this '.
+        'is not a recognized hook mode. Valid modes are: %s.',
+        $hook_mode,
+        implode(', ', array_keys($known_modes))));
+  }
+}
+
+$is_svnrevprop = ($hook_mode == 'svn-revprop');
+
+if ($is_svnrevprop) {
+  // For now, we let these through if the repository allows dangerous changes
+  // and prevent them if it doesn't. See T11208 for discussion.
+
+  $revprop_key = $argv[5];
+
+  if ($repository->shouldAllowDangerousChanges()) {
+    $err = 0;
+  } else {
+    $err = 1;
+
+    $console = PhutilConsole::getConsole();
+    $console->writeErr(
+      pht(
+        "DANGEROUS CHANGE: Dangerous change protection is enabled for this ".
+        "repository, so you can not change revision properties (you are ".
+        "attempting to edit \"%s\").\n".
+        "Edit the repository configuration before making dangerous changes.",
+        $revprop_key));
+  }
+
+  exit($err);
+} else if ($repository->isGit() || $repository->isHg()) {
   $username = getenv(DiffusionCommitHookEngine::ENV_USER);
   if (!strlen($username)) {
     throw new Exception(