xml reader: don't block attempt to read from stdin using wxr-importer import php://stdin

[drzraf]

No description provided.

[rmccue]

To fit better with how CLI apps generally work, you should pass - instead, and we should handle that if we don't already.

[drzraf]

addressed

[drzraf]

side note:
If this can be merged : wp-cli/export-command#9 ...
then it'd be possible to test transformation identity (admitting an --override flag would exist) by checking that such an operation...
$ wp export --stdout | wp wxr-importer import --override -
... implied no change on the source database.