update-source-version: Fixes, add --ignore-same-hash

--ignore-same-hash allows ignoring if the same has is used.
This gives the ability to run update-source-version multiple times to
change multiple keys.

Fix version-key, which was not being used to determine the oldVersion
variable.

If $attr.src.drvAttrs.urls is not found, look for
$attr.src.drvAttrs.url since it is being used like that in fetchgit