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.
<?xml version="1.0" encoding="utf-8" standalone="no"?>
<installer-gui-script minSpecVersion="1">
<title>Mercurial SCM</title>
<organization>org.mercurial-scm</organization>
<options customize="never" require-scripts="false" rootVolumeOnly="true" />
<welcome file="Welcome.html" mime-type="text/html" />
<license file="../../COPYING" mime-type="text/plain" />
<readme file="Readme.html" mime-type="text/html" />
<pkg-ref id="org.mercurial-scm.mercurial"
version="0"
auth="root"
onConclusion="none">mercurial.pkg</pkg-ref>
<choices-outline>
<line choice="org.mercurial-scm.mercurial"/>
</choices-outline>
<choice id="org.mercurial-scm.mercurial" visible="false">
<pkg-ref id="org.mercurial-scm.mercurial"/>
</choice>
</installer-gui-script>