Don't try to determine whether the system supports file modes
when putting the tree data in the index during checkout. The tree's
mode is canonical and did not come from stat(2) in the first place.
Created a copy of tests/resources/testrepo.git that is compatible
with the Clar sandboxing helpers.
Restructured commit test suites to use Clar sandbox helpers.
Now using typed data arrays rather than lots of macros to define test
cases.
