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.
Brad King
Felix Geyer
Daniele E. Domenichelli
Matt McCormick