Drop "allowRebase" parameter

This was only used by ReferenceBrowser, but I see no reason why we should explicitly forbid or auto-deselect it there.

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Disable "rebase after" checkbox if remote is a URL

Signed-off-by: Sven Strickroth <email@cs-ware.de>