run-tests: display the time it took to install Mercurial
It will help make people aware of this critical step and to assess the time it
takes in various options (like a CI run for example).
Set up
$ hg init repo
$ cd repo
Try to import an empty patch
$ hg import --no-commit - <<EOF
> EOF
applying patch from stdin
abort: stdin: no diffs found
[10]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate