It turns out that currently, `hg` and `which hg` can point to different things
because `hg` is an alias… This is annoying because script and piece of test are
unknowingly using the wrong `hg`.
We will fix it in the next changeset. However some test actually need to use
a real `hg` binary and not some `chg` or `rhg` equivalent. So we introduce a new
variable with the right value and we put it to us in the appropriate location.