extensions: fix wrapcommand/function of class instance
5361771f9714 changed _updatewrapper() to copy the __name__ attribute, but
not all callable objects has __name__.
Spotted by loading mq with extdiff.
$ 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