CCASE_BRANCH0_REUSE environment variable

Understand the use of the CCASE_BRANCH0_REUSE environment variable.

When clearmake evaluates a derived object for usability in the view, it compares versions of files recorded in the derived object's configuration record to versions of those same files that are in the current view. Generally, any version mismatch prevents clearmake from reusing the DO. However, if the DO used version main/123 and the view sees version main/123/branch/0, clearmake considers this to be a match. You can disable this default behavior by setting the environment variable CCASE_BRANCH0_REUSE in the shell before running clearmake.