amend: make a copy of "extra" to avoid mutating an input
I don't know of any problems this has caused, it just seems less
surprising.
Differential Revision: https://phab.mercurial-scm.org/D1575
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<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>