bug#13016: 24.3.50; Inconsistency in the way of presenting the filename in info buffers.

[Juri Linkov]

>> If using `file-name-sans-extension' is the right way
>> to fix them, then all they could be fixed with:
>
> Looks fine to me (or we could define a `Info-current-manual' subroutine
> for that).  Please, go ahead.

Installed without `Info-current-manual' (there is duplicate code in many
other places in info.el that might need refactoring as a separate task).