The fix in commit v3.8.1~4^2 (source_group: Fix TREE with root that is
not current source dir, 2017-04-20) accidentally broke support for
specifying paths relative to the source directory. Fix it and add a
test covering the case.
While at it, fix a typo in a variable name.
Fixes: #16876
Run the `Utilities/Scripts/clang-format.bash` script to update
all our C++ code to a new style defined by `.clang-format`.
Use `clang-format` version 3.8.
* If you reached this commit for a line in `git blame`, re-run the blame
operation starting at the parent of this commit to see older history
for the content.
* See the parent commit for instructions to rebase a change across this
style transition commit.
