forget: make oldest snapshot marker more strict

Now, a snapshot is only marked as oldest if it's the last in the list AND its values matches the last seen value for that bucket.

Also, updated the corresponding golden files for the tests.

internal/restic/snapshot_policy.go

@@ -256,13 +256,13 @@ func ApplyPolicy(list Snapshots, p ExpirePolicy) (keep, remove Snapshots, reason
 				if val != b.Last || nr == len(list)-1 {
 					debug.Log("keep %v %v, bucker %v, val %v\n", cur.Time, cur.id.Str(), i, val)
 					keepSnap = true
+					if val == b.Last && nr == len(list)-1 {
+						b.reason = fmt.Sprintf("oldest %v", b.reason)
+					}
 					buckets[i].Last = val
 					if buckets[i].Count > 0 {
 						buckets[i].Count--
 					}
-					if nr == len(list)-1 {
-						b.reason = fmt.Sprintf("oldest %v", b.reason)
-					}
 					keepSnapReasons = append(keepSnapReasons, b.reason)
 				}
 			}
@@ -278,10 +278,10 @@ func ApplyPolicy(list Snapshots, p ExpirePolicy) (keep, remove Snapshots, reason
 					if val != b.Last || nr == len(list)-1 {
 						debug.Log("keep %v, time %v, ID %v, bucker %v, val %v %v\n", b.reason, cur.Time, cur.id.Str(), i, val, b.Last)
 						keepSnap = true
-						bucketsWithin[i].Last = val
-						if nr == len(list)-1 {
+						if val == b.Last && nr == len(list)-1 {
 							b.reason = fmt.Sprintf("oldest %v", b.reason)
 						}
+						bucketsWithin[i].Last = val
 						keepSnapReasons = append(keepSnapReasons, fmt.Sprintf("%v %v", b.reason, b.Within))
 					}
 				}

internal/restic/testdata/policy_keep_snapshots_36

@@ -1978,7 +1978,7 @@
         "paths": null
       },
       "matches": [
-        "oldest last snapshot"
+        "last snapshot"
       ],
       "counters": {
         "last": -1

internal/restic/testdata/policy_keep_snapshots_37

@@ -2169,8 +2169,8 @@
         "paths": null
       },
       "matches": [
-        "oldest last snapshot",
-        "oldest hourly snapshot"
+        "last snapshot",
+        "hourly snapshot"
       ],
       "counters": {
         "last": -1,

internal/restic/testdata/policy_keep_snapshots_38

@@ -1708,7 +1708,7 @@
         "paths": null
       },
       "matches": [
-        "oldest hourly snapshot"
+        "hourly snapshot"
       ],
       "counters": {
         "hourly": -1

internal/restic/testdata/policy_keep_snapshots_4

@@ -1978,7 +1978,7 @@
         "paths": null
       },
       "matches": [
-        "oldest last snapshot"
+        "last snapshot"
       ],
       "counters": {
         "last": 97