Allow running fixdat without --input

[ozboss]

Is your feature request related to a problem?

igir's fixdat has the potential to replace the unfortunately unmaintained Retool in creating filtered .dat files.

Describe the solution you'd like

When running igir without --input i get the following error:

ERROR: Missing required argument for command fixdat: --input

It should not require --input as it could just simply create a .dat file with all the games that pass the filter would be given to the command.

Current workaround:
Create an empty folder and place any file into it (can just be an empty file). Provide this folder as --input to igir.

Together with #1347 I think it could replace Retool.

Additional context

No response

[emmercm]

RE: replacing Retool, I doubt Igir will come close. What made Retool special was its hand-maintained clone lists, which are going to be better than anything Igir can do with parent/clone inference.

But this is a good suggestion, this is in line with a change made to the report command a little while ago which no longer requires input files.