bundle-spec: properly identify changegroup-less bundle
It is possible to produce a bundle without changegroup. For example if we want
to only send phases or obsolescence information. However that lead to crash for
command that identifies bundle content. So we fix that.
The test will come in the next changesets, when we fix another bug preventing to
generate such bundle by hand.
$ 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