Test that pulumi new can pull the latest changes in master after a previous run

[justinvp]

We already have tests that run pulumi new, but we don't have any tests that run pulumi new from a machine that already has previously run pulumi new and has an older clone of the repo in the ~/.pulumi/templates directory.

Consider adding a test that uses the git CLI to clone the repo at the commit prior to the latest commit in master in the ~/.pulumi/templates directory, and then runs pulumi new to ensure pulumi new can successfully pull the latest changes from master.

Note: this may be less of a concern now that we have pulumi/pulumi#11175 in the CLI, which always does the equivalent of a git reset before pulling.