We call File::Temp's "tempfile" function as a class method, but it was
never designed to be called this way. Older versions seemed to
tolerate it, but as of File::Temp 0.23, it blows up like this:

  $ git svn fetch
  'tempfile' can't be called as a method at .../Git.pm line 1117.

Fix it by calling it as a regular function, just inside the File::Temp
namespace.
Signed-off-by: NH. Merijn Brand <h.m.brand@xs4all.nl>
Helped-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>