project: fix bytes/str encoding when updating git submodules

Since tempfile.mkstemp() returns a file handle in binary mode,
make sure we turn our strings into bytes before writing.

Bug: https://crbug.com/gerrit/12043
Change-Id: I3e84d595e84b8bc12a1fbc7fd0bb3ea0ba2832b0
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254393


Reviewed-by: Michael Mortensen <mmortensen@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Mike Frysinger <vapier@google.com>
(cherry picked from commit 163d42eb)