lib/db: Don't whack blocks when putting truncated file (#6434)

As of the latest database checker we are again putting files without
blocks. I'm not 100% convinced that's a great idea, but we also do it
for ignored files apparently so it looks like we probably should support
it. This adds an escape hatch that must be manually enabled...

lib/db/transactions.go

@@ -432,13 +432,19 @@ func (t readWriteTransaction) close() {
 	t.WriteTransaction.Release()
 }
 
-func (t readWriteTransaction) putFile(fkey []byte, fi protocol.FileInfo) error {
+// putFile stores a file in the database, taking care of indirected fields.
+// Set the truncated flag when putting a file that deliberatly can have an
+// empty block list but a non-empty block list hash. This should normally be
+// false.
+func (t readWriteTransaction) putFile(fkey []byte, fi protocol.FileInfo, truncated bool) error {
 	var bkey []byte
 
-	// Always set the blocks hash when there are blocks.
+	// Always set the blocks hash when there are blocks. Leave the blocks
+	// hash alone when there are no blocks and we might be putting a
+	// "truncated" FileInfo (no blocks, but the hash reference is live).
 	if len(fi.Blocks) > 0 {
 		fi.BlocksHash = protocol.BlocksHash(fi.Blocks)
-	} else {
+	} else if !truncated {
 		fi.BlocksHash = nil
 	}