test: use bundle2 for exchange in test-push-warn
Using bundle2 for exchange unlocks the usage of changegroup version 2. This
version of the changegroup held more information (delta base) that result in
a small increase in content size (20 bytes x 2 changes).
os = <unloaded module 'os'>
os.system = <built-in function system>
os = <module 'os' from '?'>
util = <unloaded module 'util'>
util.system = <function system at 0x?>
util = <module 'mercurial.util' from '?'>
util.system = <function system at 0x?>
fred = <unloaded module 're'>
re = <unloaded module 'sys'>
fred = <unloaded module 're'>
fred.sub = <function sub at 0x?>
fred = <proxied module 're'>
re = <unloaded module 'sys'>
re.stderr = <open file '<whatever>', mode 'w' at 0x?>
re = <proxied module 'sys'>
node = <module 'mercurial.node' from '?'>