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>

line-log.c

@@ -115,6 +115,8 @@ static void sort_and_merge_range_set(struct range_set *rs)
 	qsort(rs->ranges, rs->nr, sizeof(struct range), range_cmp);
 
 	for (i = 0; i < rs->nr; i++) {
+		if (rs->ranges[i].start == rs->ranges[i].end)
+			continue;
 		if (o > 0 && rs->ranges[i].start <= rs->ranges[o-1].end) {
 			if (rs->ranges[o-1].end < rs->ranges[i].end)
 				rs->ranges[o-1].end = rs->ranges[i].end;

t/t4211-line-log.sh

@@ -67,7 +67,8 @@ test_bad_opts "-L :foo:b.c" "no match"
 # There is a separate bug when an empty -L range is the first -L encountered,
 # thus to demonstrate this particular bug, the empty -L range must follow a
 # non-empty -L range.
-test_expect_failure '-L {empty-range} (any -L)' '
+test_expect_success '-L {empty-range} (any -L)' '
+	n=$(expr $(cat b.c | wc -l) + 1) &&
 	n=$(expr $(wc -l <b.c) + 1) &&
 	git log -L1,1:b.c -L$n:b.c
 '