The code to load .netrc file copies it to a temporary file
named $TEMP/XXXXXXXXXgerrit_util/netrc before loading it
to workaround crbug.com/664664.

However, using a netrc file is deprecated (in favor of a
gitcookies file instead) so most of the time, the file is
empty. Instead of creating an empty file, load data from
/dev/null (named os.devnull).

This avoid leaving an empty file lying around (since the
temporary file is not deleted), and avoid race-conditions
on Windows when multiple processes/threads try to create
the same file.

Bug: none
Change-Id: I873b72402038ca3f0d32387353dc89b731aaa927
Reviewed-on: https://chromium-review.googlesource.com/1132998
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Ryan Tseng <hinoka@chromium.org>