Allow gclient sync to overwrite untracked files when moving detached git HEAD.

gclient_scm.py

Index: gclient_scm.py
diff --git a/gclient_scm.py b/gclient_scm.py
index 67d68674e04fd5f0c42084529551a1fa35c181f5..3394fd825e9e9ac8a77bf8af5bb0089aff2b7a9a 100644
--- a/gclient_scm.py
+++ b/gclient_scm.py
@@ -486,7 +486,12 @@ class GitWrapper(SCMWrapper):
       if self._Capture(['rev-list', '-n', '1', 'HEAD']) == revision:
         self.Print('Up-to-date; skipping checkout.')
       else:
-        self._Capture(['checkout', '--quiet', '%s' % revision])
+        # 'git checkout' may need to overwrite existing untracked files. Allow
+        # it only when nuclear options are enabled.
+        if options.force and options.delete_unversioned_trees:

[Vadim Sh., 2014/06/20 06:46:46]

The more natural combination would be "--force --reset", but 'gclient sync' step
on bots is not using --reset, and enabling --reset there would make it a lot
slower (I think). So I choose less natural combination of '--force
--delete_unversioned_trees' that's already passed by gclient sync step.

In some sense force git checkout does delete unversioned trees... sort of.

+          self._Capture(['checkout', '--force', '--quiet', '%s' % revision])
+        else:
+          self._Capture(['checkout', '--quiet', '%s' % revision])
       if not printed_path:
         self.Print('_____ %s%s' % (self.relpath, rev_str), timestamp=False)
     elif current_type == 'hash':