COMP: Except Git's status messages from CTest reporting

Git prints some of its normal status messages to stderr. This output can
be mistaken for warnings by CTest, particularly if the target Git
repository uses the word "warning" (for instance in a branch name).

This modifies the regex CTEST_CUSTOM_WARNING_EXCEPTION to also catch
and except the messages printed to stderr by "git clone" and
"git checkout" (to a detached HEAD), which occur when using Git to
fetch an ExternalModule.

A patch is also being submitted upstream to CMake to fix the same issue.

Change-Id: I7208bd811dd3a6c4f06a10ced5ef2caecaa30452
Merge topic 'DetachedHEAD-Warning2'

71d18597 COMP: Except Git's status messages from CTest reporting