[RFE] Teach git textconv to support %f

The filter structure provides a mechanism for providing the working directory's file name path to a filter using a %f argument. This
request is to teach the textconv mechanism to support the same capability.

The use case comes from a complex content renderer that needs to know what the original file name is, so as to be able to find
additional content, by name, that describes the file (base name+different extension).

If this is considered a good idea, I would be happy to implement this but need a pointer or two of where to look in the code to make
it happen.

Thanks,
Randall


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