ExternalProject: Prevent the noisy detached head messages on checkout

When checking out a specific commit by its git hash, git will output
a very noisy message about checking out a detached HEAD. This
is not particularly helpful for us here and makes the output overall
quite verbose. Add a git config setting to prevent it.

Modules/ExternalProject.cmake

@@ -2671,6 +2671,10 @@ function(_ep_add_download_command name)
     get_property(git_progress TARGET ${name} PROPERTY _EP_GIT_PROGRESS)
     get_property(git_config TARGET ${name} PROPERTY _EP_GIT_CONFIG)
 
+    # Make checkouts quiet when checking out a git hash (this avoids the
+    # very noisy detached head message)
+    list(PREPEND git_config advice.detachedHead=false)
+
     # For the download step, and the git clone operation, only the repository
     # should be recorded in a configured RepositoryInfo file. If the repo
     # changes, the clone script should be run again. But if only the tag