range-set: satisfy non-empty ranges invariant

range-set: satisfy non-empty ranges invariant

range-set invariants are: ranges must be (1) non-empty, (2) disjoint,
(3) sorted in ascending order.

During processing, various range-set utility functions break the
invariants (for instance, by adding empty ranges), with the
expectation that a finalizing sort_and_merge_range_set() will restore
sanity.

sort_and_merge_range_set(), however, neglects to fold out empty
ranges, thus it fails to satisfy the non-empty constraint. Subsequent
range-set functions crash or throw an assertion failure upon
encountering such an anomaly. Rectify the situation by having
sort_and_merge_range_set() fold out empty ranges.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Acked-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>