[PATCH] Fix #3390 Relative externals not updated during switch, for backport to 1.6.x

Hi Stefan!

This is supposed to be a backport. It is diffed against 1.6.x.

make check passed.

[[[
Fix issue #3390, relative externals not updated during switch.

* subversion/libsvn_client/switch.c
  (svn_client__switch_internal): Pass in a traversal_info to
    svn_wc_crawl_revision4().

* subversion/tests/cmdline/externals_tests.py
  (switch_relative_external): New.
  (test_list): Add new test.

Patch by: Daniel Näslund <daniel{_AT_}longitudo.com>
]]]

Daniel