The file context created in add_file() is supposed to survive beyond the
end of function, so it needs its own copy of copy_path.

The failure scenario was observed in git-svn usage, with errors like

perl: subversion/libsvn_subr/dirent_uri.c:2489:
      svn_fspath__skip_ancestor:
      Assertion `svn_fspath__is_canonical(child_fspath)' failed.

* subversion/libsvn_ra_serf/commit.c
  (add_file): All members of the new_file struct shall have
              the same lifetime as the struct itself.

Path by: Roman Kagan (rkagan<{at}>mail.ru)

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1553376 13f79535-47bb-0310-9956-ffa450edef68