source_group: Restore TREE support for relative paths

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

1 files changed, 4 insertions, 0 deletions

diff --git a/Tests/SourceGroups/tree_prefix_bar.c b/Tests/SourceGroups/tree_prefix_bar.c
new file mode 100644
index 0000000..bd98476
--- /dev/null
+++ b/Tests/SourceGroups/tree_prefix_bar.c
@@ -0,0 +1,4 @@
+int tree_prefix_bar(void)
+{
+  return 66;
+}