wcfs: zdata: ΔFtail tests: Fix/Adjust debug dump for computed blkRevAt

- put into if block to avoid collision with already-defined-elsewhere blkv
- show revisions in symbolic form

Noticed while working on recent change to allow ΔFtail/ΔBtail
point-queries with at=tail.

wcfs/internal/zdata/δftail_test.go

@@ -386,6 +386,7 @@ func testΔFtail(t_ *testing.T, testq chan ΔFTestEntry) {
 		}
 		blkRevAt[commit.At] = blkRev
 		/*
+		if false {
 			fmt.Printf("blkRevAt[@%s]:\n", commit.AtSymb())
 			blkv := []int64{}
 			for blk := range blkRev {
@@ -395,7 +396,8 @@ func testΔFtail(t_ *testing.T, testq chan ΔFTestEntry) {
 				return blkv[i] < blkv[j]
 			})
 			for _, blk := range blkv {
-				fmt.Printf("  #%d:  %v\n", blk, blkRev[blk])
+				fmt.Printf("  #%d:  @%s\n", blk, t.AtSymb(blkRev[blk]))
+			}
 		}
 		*/