strbuf `filter' API

  Ooookay,

  I wasn't more thrilled about the previous patch than linus is. Here is
a possible API to hide the muck in the strbuf.c module, and have a way
nicer to read code in convert.c.

  It remais as efficient as the previous code, tries to reuse the buffer
as much as possible to avoid reallocations, and deal with even more
cases (it's possible to have the "source" buffer be inside the
destination strbuf now, which could sometimes help a lot).

  I based this on top of master (because it's where my previous patch
was) but it should apply on next properly. It depends upon the previous
patch though.

  I'd say that the previous patch should go in master ASAP, and that
those two could be considered for next (having merged master in it first
btw).

Cheers,