Update git_blob_create_fromchunks callback behavr

The callback to supply data chunks could return a negative value
to stop creation of the blob, but we were neither using GIT_EUSER
nor propagating the return value.  This makes things use the new
behavior of returning the negative value back to the user.
Fix checkout notify callback docs and tests

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.
Update clone doc and tests for callback return val

Clone callbacks can return non-zero values to cancel the clone.
This adds some tests to verify that this actually works and updates
the documentation to be clearer that this can happen and that the
return value will be propagated back by the clone function.
Try a test that won't assert on Linux