This is required to avoid the need to clobber the bots when moving a directory 
to deps/. Currently, the directory in question is likely to remain in the 
working copy, despite having been removed, due to the presence of untracked 
files. This causes the checkout from deps/ to fail. 

With this change, when both --reset and --delete_unversioned_trees are 
specified, the the directory in question will be removed from the working copy, 
thereby allowing the copy in deps/ to be checked out correctly. 

Note that untracked directories which are explicitly ignored (ie in .gitignore 
or svn:ignore) will not be removed. 

Note that this was previously landed in http://codereview.chromium.org/9348054 
but reverted due to problems with symlinks in the chromeos build. 

BUG=112887, chromium-os:20759

Review URL: http://codereview.chromium.org/9404014

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@122300 0039d316-1c4b-4281-b951-d872f2087c98