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.
Any well-behaved program should write a descriptive message to the
reflog whenever it updates a reference. Let's make this more prominent
by removing the version without the reflog parameters.
The checkout notify callback behavior on non-zero return values
was not being tested. This adds tests, fixes a bug with positive
values, and clarifies the documentation to make it clear that the
checkout can be canceled via this mechanism.
