Fix an issue where "git" may be unable to read a temporary file in Diffusion

Summary:
Ref T13673. After the changes in that task, we may execute "git config -l ..." as a user other than the user we used to write this temporary file.

Use "--file -" to pass the data instead, avoiding use of temporary files. This makes us agnostic to filesystem permissions.

Test Plan: Viewed a Git repository with submodules in Diffusion with "ssh.user" configured as a user relatively isolated from the webserver user.

Maniphest Tasks: T13673

Differential Revision: https://secure.phabricator.com/D21759