bundle2: introduce `replycaps` part for on-demand reply
The bundle2 processing does not create a bundle2 reply by default anymore. It
is only done if the client requests it with a `replycaps` part. This part is
called `replycaps` as it will eventually contain data about which bundle2
capabilities are supported by the client.
We have to add a flag to the test command to control whether a reply is
generated or not.
$ hg init
$ echo a > a
$ hg ci -qAm 'add a'
$ hg init subrepo
$ echo 'subrepo = http://example.net/libfoo' > .hgsub
$ hg ci -qAm 'added subrepo'
$ hg up -qC 0
$ echo ax > a
$ hg ci -m 'changed a'
created new head
$ hg up -qC 1
$ cd subrepo
$ echo b > b
$ hg add b
$ cd ..
Should fail, since there are added files to subrepo:
$ hg merge
abort: uncommitted changes in subrepository 'subrepo'
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