Arc cover handles a file that is added and modified correctly.

Summary:
Modifying a new file and running 'arc cover' before committing the
modifications confused arc. The problem was that 'unstaged' status
erased 'added' status and this caused problems.

Test Plan:
Tested that arc cover ignores added files when they are modified, but
old files that are modified are still handled correctly by arc cover.

Reviewed By: epriestley
Reviewers: epriestley
CC: jungejason, epriestley
Differential Revision: 122

src/repository/api/git/ArcanistGitAPI.php

@@ -200,7 +200,9 @@ class ArcanistGitAPI extends ArcanistRepositoryAPI {
       if (strlen($stdout)) {
         $stdout = explode("\n", $stdout);
         foreach ($stdout as $file) {
-          $files[$file] = self::FLAG_UNSTAGED;
+          $files[$file] = isset($files[$file])
+            ? ($files[$file] | self::FLAG_UNSTAGED)
+            : self::FLAG_UNSTAGED;
         }
       }