Prevent git hardlinks when cloning locally

Review Request #10172 — Created Sept. 22, 2018 and submitted — Latest diff uploaded

reviewbot, students

When a ReviewBot RepositoryTool clones, it runs git clone with the
--local flag, which among other things, saves space by using
hardlinks. Since attempts to clone into /tmp, which
is often a ramdisk, it's considered a separate device. Hardlinks don't
support this, so the tool fails to run with:

fatal: failed to create link <path> Invalid cross-device link

This patch adds --no-hardlinks to the clone which allows us to still
use /tmp, but it doesn't fail on local repositories.

I've tested this by adding a new review request after patching which
correctly clones and runs my own reviewbot plugin.