The purpose of the `update` step is to run an update on each build
(subject to `UPDATE_DISCONNECTED`). This is done for version-controlled
source directories. We should do it for a custom `UPDATE_COMMAND` too.
In particular, when `UPDATE_DISCONNECTED` is used we expect the
`skip-update` step to exist.
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.
This prepares the numberous tests that occur in the ExternalProjectUpdate
test. The tests were passing previously because a fresh build was not performed.
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.
