compat: add a fixed version of mergecopies for hg<=4.3
In hg<=4.3, copies._fullcopytracing() does not exist and we have to wrap
copies.mergecopies() to fix the AssertionError coming from core.
$ cat >> $HGRCPATH <<EOF
> [ui]
> logtemplate={rev}:{node|short}[{bookmarks}] ({obsolete}/{phase}) {desc|firstline}\n
> [extensions]
> EOF
$ echo "evolve=$(echo $(dirname $TESTDIR))/hgext3rd/evolve/" >> $HGRCPATH
$ mkcommit() {
> echo "$1" > "$1"
> hg add "$1"
> hg ci -m "add $1"
> }
$ hg init repo
$ cd repo
$ mkcommit a
$ mkcommit b
test disabling commands
$ cat >> .hg/hgrc <<EOF
> [experimental]
> evolution=createmarkers
> allowunstable
> exchange
> EOF
$ hg prune | head -n 2
hg: unknown command 'prune'
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