internal/manifest: fix Overlaps L0 expansion with exclusive-end

When Version.Overlaps is called for L0, the overlap window is
iteratively expanded until stable. In #1432, the Overlaps function was
adjusted to allow specifying that the end bound should be considered
exclusive. However, #1432 failed to update the exclusivity of the end
bound when the range widened. This improperly excluded files with
largest keys that exactly equaled the new widened end bound.

This commit also transforms the TestOverlaps test into a datadriven
test, introducing a few helpers for parsing the DebugString output of a
Version.

Fix #1459.

internal/base/internal.go

@@ -359,3 +359,16 @@ type prettyInternalKey struct {
 func (k prettyInternalKey) Format(s fmt.State, c rune) {
 	fmt.Fprintf(s, "%s#%d,%s", k.formatKey(k.UserKey), k.SeqNum(), k.Kind())
 }
+
+// ParsePrettyInternalKey parses the pretty string representation of an
+// internal key. The format is <user-key>#<seq-num>,<kind>.
+func ParsePrettyInternalKey(s string) InternalKey {
+	x := strings.FieldsFunc(s, func(c rune) bool { return c == '#' || c == ',' })
+	ukey := x[0]
+	kind, ok := kindsMap[x[2]]
+	if !ok {
+		panic(fmt.Sprintf("unknown kind: %q", x[2]))
+	}
+	seqNum, _ := strconv.ParseUint(x[1], 10, 64)
+	return MakeInternalKey([]byte(ukey), seqNum, kind)
+}