convert: save hg revision during a convert
This breaks hg->hg hash roundtripping, but that was fragile, didn't work
reliably, and hence arguably of little value.
#!/bin/sh
hg init a
echo a > a/a
hg --cwd a ci -A -m a
hg clone a b
echo b > b/b
hg --cwd b ci -A -m b
chmod 100 a/.hg/store
hg --cwd b push ../a
chmod 700 a/.hg/store