bundle2: fix broken compression engine assertion
bundletype() is a function, so it needs to be called, and it is
documented to return a 2-tuple. This code is untested, so that's why
we haven't noticed the bad assertion.
Differential Revision: https://phab.mercurial-scm.org/D5155
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a