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).
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls -A
.hg
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls -A
.hg
a