Only send useful oldid while switching to wikitext editor if it's specifically requested

Usually we can just send 0. Sending anything else makes PostEdit think we're restoring an old revision.

Bug: 65269
Change-Id: I60454a7a4ea3f6c7cef4c707da3016dd0ec29b88
Merge "Only send useful oldid while switching to wikitext editor if it's specifically requested"