bundle: push chunkbuffer down into decompress
We replace the fixup iterator with a file-like object so that
uncompressed file streams can be passed end to end through the stack.
$ hg init a
$ hg clone a b
updating to branch default
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd a
$ echo '[paths]' >> .hg/hgrc
$ echo 'dupe = ../b' >> .hg/hgrc
$ hg in dupe
comparing with .*/test-paths.t/b
no changes found
[1]
$ cd ..
$ hg -R a in dupe
comparing with .*/test-paths.t/b
no changes found
[1]
$ true