When calling `git ls-files -m` to list modified files, use --with-tree=HEAD

When calling `git ls-files -m` to list modified files, use --with-tree=HEAD
so that differences from the most recent commit are given instead of
differences from the index which is used for commit staging purposes.
This is required since otherwise files that have been added via `git add`
(such as echangelog does with the ChangeLog) won't show up in the list
of modified files due to being unmodified relative to the index. Thanks
to Christian Ruppert (idl0r) for reporting.

svn path=/main/trunk/; revision=12140