internal/report: add field unexcluded to YAML

Record the reason a report was previously excluded when
unexcluding it. This will allow us to take this info into account
when deciding the priority of new reports.

Change-Id: I6ad08f28ca7f9bec78280f30db35b0b6546085db
Reviewed-on: https://go-review.googlesource.com/c/vulndb/+/592776
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>

cmd/vulnreport/unexclude.go

@@ -72,6 +72,7 @@ func (u *unexclude) run(ctx context.Context, input any) (err error) {
 	if err != nil {
 		return err
 	}
+	r.Unexcluded = oldR.Excluded
 	if err := u.write(ctx, r); err != nil {
 		return err
 	}

internal/report/report.go

@@ -244,6 +244,10 @@ type Report struct {
 	SourceMeta *SourceMeta `yaml:"source,omitempty"`
 
 	ReviewStatus ReviewStatus `yaml:"review_status,omitempty"`
+
+	// (For unexcluded reports) The reason this report was previously
+	// excluded. Not published to OSV.
+	Unexcluded ExcludedReason `yaml:"unexcluded,omitempty"`
 }
 
 // This wrapper is needed so we can define YAML functions on this type.