target:add: estimate the time left only once enough of the index is in

The first index file also carries the time to connect, which made an S3
location say "about 642 min left" for something done within a minute.

lib/Command/TargetAdd.php

@@ -88,19 +88,21 @@ HELP);
 	 * @return callable(int, int): void
 	 */
 	private static function progress(OutputInterface $output): callable {
-		$start = microtime(true);
 		$shown = 0.0;
+		$first = null; // [time, files done] when the first files were in
 		$inPlace = $output->isDecorated();
-		return static function (int $done, int $total) use ($output, $start, &$shown, $inPlace): void {
+		return static function (int $done, int $total) use ($output, &$shown, &$first, $inPlace): void {
 			$now = microtime(true);
+			if ($first === null && $done > 0) {
+				$first = [$now, $done];
+			}
 			if ($done < $total && $now - $shown < ($inPlace ? 1 : 30)) {
 				return;
 			}
 			$shown = $now;
 			$line = sprintf('Reading the index: %s/%s files (%d%%)', number_format($done), number_format($total), intdiv(100 * $done, max(1, $total)));
-			$elapsed = $now - $start;
-			if ($done < $total && $done > 0 && $elapsed >= 5) {
-				$left = (int)ceil($elapsed / (float)$done * (float)($total - $done) / 60.0);
+			$left = $first === null ? null : self::minutesLeft($now - $first[0], $done - $first[1], $total - $done, $total);
+			if ($left !== null) {
 				$line .= $left <= 1 ? ', less than a minute left' : sprintf(', about %d min left', $left);
 			}
 			if ($inPlace) {
@@ -110,4 +112,17 @@ HELP);
 			}
 		};
 	}
+
+	/**
+	 * Minutes left for reading the index, from the rate since the first files were in: the first
+	 * file also carries the time to connect, which made an S3 location with 4,568 index files
+	 * say "about 642 min left" for something done within a minute. No estimate until 5% of the
+	 * files, or 50 files, came in after that, and at least 5 seconds passed.
+	 */
+	public static function minutesLeft(float $elapsed, int $done, int $left, int $total): ?int {
+		if ($left <= 0 || $done <= 0 || $elapsed < 5.0 || ($done < 50 && $done * 20 < $total)) {
+			return null;
+		}
+		return (int)ceil($elapsed / (float)$done * (float)$left / 60.0);
+	}
 }