(experimental) Work around a Windows escaping issue and security conecern in "hg cat --output ..."

Summary:
See PHI904. Ref T13210. Ref T13209. Currently, we have an `hg cat` construction which attempts to pass a literal `%p` to Mercurial. This fails because you can't pass `%` through `%s` outside of `wilds`.

It also uses `%C` to pass a list of file paths. This is broadly unsafe and can cause command execution if you modify a file named, e.g., `; rm -rf xyz` or similar. I think it would be difficult to turn this into an attack but it's fairly bad. This dates from D5144 in 2013.

Test Plan: With this patch, created D19757 which has valid binary data (see F5962134).

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13210, T13209

Differential Revision: https://secure.phabricator.com/D19758

src/repository/api/ArcanistMercurialAPI.php

@@ -460,12 +460,16 @@ final class ArcanistMercurialAPI extends ArcanistRepositoryAPI {
       Filesystem::createDirectory(dirname($tmppath), 0755, true);
     }
 
+    // NOTE: The "%s%%p" construction passes a literal "%p" to Mercurial,
+    // which is a formatting directive for a repo-relative filepath. The
+    // particulars of the construction avoid Windows escaping issues. See
+    // PHI904.
+
     list($err, $stdout) = $this->execManualLocal(
-      'cat --rev %s --output %s -- %C',
+      'cat --rev %s --output %s%%p -- %Ls',
       $revision,
-      // %p is the formatter for the repo-relative filepath
-      $tmpdir.'/%p',
-      implode(' ', $paths));
+      $tmpdir.DIRECTORY_SEPARATOR,
+      $paths);
 
     $filedata = array();
     foreach ($paths as $path) {