Add an (int) $resumeAt argument for ProgressBar#start(), so that the progress bar is initialized at a specific progress level, with a new property so that getEstimated() and getRemaining() report the right number.

As pointed out on a recently posted SO question), when working on a longish task one may want to create a progress bar for a resumed task.

Calling advance() works to increase the progress bar counter, but the results of getRemaining() and getEstimated() will be "broken", since whatever steps one "skipped" will be considered to have taken 0 seconds. For a longer running task, the estimation will be very inaccurate, and will remain so for a long while. Using the example code from the linked question:

$itemCount = 1_000_000;
$startItem = 150_000;

$progressBar = new ProgressBar($output, $itemCount);
$progressBar->setFormat(
    $progressBar->getFormatDefinition(ProgressBar::FORMAT_DEBUG)
);

$progressBar->start();

if ($startItem !== 0) {
    // unfortunately taken into account in ETA calculation
    $progressBar->advance($startItem);
}

for ($i = $startItem; $i < $itemCount; $i++) {
    usleep(50_000);
    $progressBar->advance();
}

This will output this to begin with:

150038/1000000 [====>-----------------------]  15% 2 secs/13 secs 20.0 MiB

The estimated time will keep growing as the tasks progresses, but the estimation won't be remotely useful for a long while.

This PR adds a $resumeAt parameter to ProgresBar#start(), so that's possible to write:

$bar = new ProgressBar($output, 1_200_000, 0);
$bar->start(null, 300_000);

And calls to $bar->getEstimated() and $bar->getRemaining() will return sane results (and calling the initial ->advance() would be no longer necessary).

TODO:

• submit changes to the documentation (if the PR is well received)