fix: support downgrade/upgrade for unknown versions

cli/app/rollback.go

@@ -62,10 +62,6 @@ data beforehand - see "abra app backup <app>" for more.
 			logrus.Fatal(err)
 		}
 
-		if deployedVersion == "" {
-			logrus.Fatalf("failed to determine version of deployed '%s'", app.Name)
-		}
-
 		if !isDeployed {
 			logrus.Fatalf("'%s' is not deployed?", app.Name)
 		}
@@ -76,22 +72,30 @@ data beforehand - see "abra app backup <app>" for more.
 		}
 
 		var availableDowngrades []string
-		for _, version := range versions {
-			parsedDeployedVersion, err := tagcmp.Parse(deployedVersion)
-			if err != nil {
-				logrus.Fatal(err)
-			}
-			parsedVersion, err := tagcmp.Parse(version)
-			if err != nil {
-				logrus.Fatal(err)
-			}
-			if parsedVersion != parsedDeployedVersion && parsedVersion.IsLessThan(parsedDeployedVersion) {
-				availableDowngrades = append(availableDowngrades, version)
-			}
+		if deployedVersion == "" {
+			deployedVersion = "unknown"
+			availableDowngrades = versions
+			logrus.Warnf("failed to determine version of deployed '%s'", app.Name)
 		}
 
-		if len(availableDowngrades) == 0 {
-			logrus.Fatal("no available downgrades, you're on latest")
+		if deployedVersion != "unknown" {
+			for _, version := range versions {
+				parsedDeployedVersion, err := tagcmp.Parse(deployedVersion)
+				if err != nil {
+					logrus.Fatal(err)
+				}
+				parsedVersion, err := tagcmp.Parse(version)
+				if err != nil {
+					logrus.Fatal(err)
+				}
+				if parsedVersion != parsedDeployedVersion && parsedVersion.IsLessThan(parsedDeployedVersion) {
+					availableDowngrades = append(availableDowngrades, version)
+				}
+			}
+
+			if len(availableDowngrades) == 0 {
+				logrus.Fatal("no available downgrades, you're on latest")
+			}
 		}
 
 		// FIXME: jeezus golang why do you not have a list reverse function