bundle2: don't advertise stream bundle2 capability when feature disabled
The server.uncompressed config option can be used to disable streaming
clones. While the top-level capability to advertise streaming clone
support isn't advertised when this option is set, we were still sending
the bundle2-level capabilities advertising support for stream parts.
It makes sense to not advertise that support when streaming clones
are globally disabled.
If the structure of the new code seems a bit odd, it is because we'll
have to further tweak the behavior in commit(s) ahead.
Differential Revision: https://phab.mercurial-scm.org/D1929
$ hg init
$ echo 123 > a
$ hg add a
$ hg commit -m "first" a
$ mkdir sub
$ echo 321 > sub/b
$ hg add sub/b
$ hg commit -m "second" sub/b
$ cat sub/b
321
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cat sub/b 2>/dev/null || echo "sub/b not present"
sub/b not present
$ test -d sub || echo "sub not present"
sub not present