Commit 0aea435aa1 (ExternalProject: Provide choice of
git update strategies, 2020-02-12) added the git update
strategies, but the CHECKOUT strategy was not handling
remote refs correctly. The local ref would be checked out
instead and no warning or error would have been emitted.
The test that should have caught this was also malformed
and did not actually move the local master branch as intended.
The performance feature of only performing a git fetch when needed
during the ExternalProject update step is verified during the test.
A fetch is identified by removing the FETCH_HEAD file and checking for
its reincarnation.
Tests are added for UPDATE_COMMAND to ensure it is working properly. Testing
infrastructure is added along with tests for Git, but tests for other version
control systems could easily be added in the future.
