Git.pm: call tempfile from File::Temp as a regular function

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: H. Merijn Brand <h.m.brand@xs4all.nl>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
fix --use-log-author making email without uuid in commits without "From:" field

    good: Author: tigrisc <tigrisc@8e71dc83-d81e-6649-80f2-80b843a9b2be>
    bad : Author: tigrisc <tigrisc>