bundle2: allow compressed bundle
This changeset adds support for a 'compression' parameter in bundle2 streams.
When set, it controls the compression algorithm used for the payload part of the
bundle2.
There is currently no usage of this except in tests.
#require execbit
$ hg init repo
$ cd repo
$ echo foo > foo
$ chmod 644 foo
$ hg ci -qAm '644'
$ chmod 755 foo
$ hg ci -qAm '755'
reverting to rev 0
$ hg revert -a -r 0
reverting foo
$ hg st
M foo
$ hg diff --git
diff --git a/foo b/foo
old mode 100755
new mode 100644
$ cd ..