media_upload_form_handler runs twice when editing attachment from "From Computer" media upload tab (browser uploader)

[jeffstieler]

After uploading a new media item "From Computer" using the browser uploader, you are presented with a form to edit its fields.

Clicking "Save all changes" from this form causes media_upload_form_handler to run, then media_upload_gallery (presumably to redirect to the gallery tab), which then causes media_upload_form_handler to run again.

This can cause problems for edit_attachment actions that only run once and alter the posts table row for that attachment (since the subsequent media_upload_form_handler could overwrite whatever changes were made in the action if they don't match whats in $_POST).

If the only reason media_upload_gallery is being called from the other type upload handler is to redirect to the gallery tab.. perhaps this patch is a better approach.

[coffee2code]

Patch no longer applies cleanly against trunk as of r21435.

[antpb]

Looks like there is only one instance of this now that needs the change. Patch refreshed below.

[wonderboymusic]

In 34003:

In wp_media_upload_handler(), replace a lingering instance of media_upload_gallery() with wp_iframe(...)

Props jeffstieler, antpb.
Fixes #17812.