writebundle: add a compression argument for the bundle2 case
Bundle2 compression is more complex than the bundle1 one. Therefore it
is handled by the bundler itself. Moreover, on-disk bundle2 will
probably have a large number of flavors so simply adding a new "format"
for it does not seems the way to go.
This will be used in the next changeset to compress bundle2 strip backup.
$ hg init
$ $PYTHON -c 'file("a", "wb").write("confuse str.splitlines\nembedded\rnewline\n")'
$ hg ci -Ama -d '1 0'
adding a
$ echo clean diff >> a
$ hg ci -mb -d '2 0'
$ hg diff -r0 -r1
diff -r 107ba6f817b5 -r 310ce7989cdc a
--- a/a Thu Jan 01 00:00:01 1970 +0000
+++ b/a Thu Jan 01 00:00:02 1970 +0000
@@ -1,2 +1,3 @@
confuse str.splitlines
embedded\r (no-eol) (esc)
newline
+clean diff