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).
skip ignored directories if -i or --all not specified
$ hg init t
$ cd t
$ echo 'ignored' > .hgignore
$ hg ci -qA -m init -d'2 0'
$ mkdir ignored
$ ls
ignored
$ hg purge -v --no-confirm
$ ls
ignored