Merge lp://qastaging/~piastucki/bzr-xmloutput/fix-551351-ignored-in-symlink-root into lp://qastaging/bzr-xmloutput

This patch fixes the way ignored files are handled in case the branch is access via symlink rather than a real path.

Invoking xmlls in the following way
bzr xmlls --ignored symlink

Results in an error (see traceback below) because the code first concatenates the symlink with the relative path of the ignored file and then tries to get the relative path by calling tree.relpath(fp). tree.relpath(fp) cannot find the relative path as bzr seems to expand symlinks so that the branch root (tree.basedir) contains the actual path.
The fix changes the order of pattern lookup and path concatenation and completely removes the need to call tree.relpath(fp).

Traceback:
0.102 Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/xmloutput/xml_errors.py", line 69, in xml_error_handling
    return func(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/xmloutput/cmds.py", line 385, in run
    lsxml.show_ls_xml(self.outf, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/xmloutput/lsxml.py", line 89, in show_ls_xml
    pattern = '<pattern>%s</pattern>' % _escape_cdata(pat)
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/xmloutput/writer.py", line 13, in _escape_cdata
    text = text.replace("&", "&amp;")
AttributeError: 'NoneType' object has no attribute 'replace'