Enable test logs with environment variable
Fix issue with checking out from Git cache: #11

Checking out from local clone of remote Git repositories wasn't working
as expected. The first time, the working copy would be updated from the
tag because the clone would be checking out the tag. Subsequent times,
with a different (empty) working directory, Git would treat all the
files as deleted in the working copy, and not update them. Using `git
checkout -f <tag>` instead of `git checkout <tag>` fixed this, but
highlights a race condition that probably existed for reading from local
Git clones. Synchronized read access to Git repositories for now, until
we can work out how to use Git properly for this.
Check out files from Git mirrors without moving HEAD

Avoid moving HEAD around in Git mirrors: see #11