RFD: concatening textconv filters

[Michael J Gruber <git@drmicha.warpmail.net>]

During my day-to-day UGFWIINIT I noticed that we don't do textconv
iteratively. E.g.: I have a file

SuperSecretButDumbFormat.pdf.gpg

and textconv filters with attributes set for *.gpg and *.pdf (using
"gpg" resp. "pdftotext"). For Git, the file has only the "gpg"
attribute, of course. In this case, I would have wanted to pass the gpg
output through pdftotext.

Now, I can set up an extra filter "gpgtopdftotext" for *.pdf.gpg (hoping
I get the ordering in .gitattributes right), of course, but wondering
whether we could and should support concatenating filters by either

- making it easy to request it (say by setting
"filter.gpgtopdftotext.textconvpipe" to a list of textconv filter names
which are to be applied in sequence)

or

- doing it automatically (remove the pattern which triggered the filter,
and apply attributes again to the resulting pathspec)

Maybe it's just not worth the effort. Or a nice GSoC project ;)

Michael