discovery-helper: reflect argument value in the name of the results

It is common to create multiple pairs of repositories using different argument
values. Recording the argument value in the results names has two main
advantages:

* It is easy to remember the value used to create a pair,
* It get simpler to create multiple pair at the same time from the same source.

Previously, running: `./discovery-helper.sh pypy 50 10` would create a
`pypy-left` and `pypy-right` repository. Now it will create `pypy-50h-10d-left`
and `pypy-50h-10d-right`.

contrib/perf-utils/discovery-helper.sh

@@ -42,8 +42,8 b' shift'
 depth="$1"
 shift
 
-leftrepo="${repo}-left"
-rightrepo="${repo}-right"
+leftrepo="${repo}-${nbheads}h-${depth}d-left"
+rightrepo="${repo}-${nbheads}h-${depth}d-right"
 
 left="first(sort(heads(all()), 'desc'), $nbheads)"
 right="last(sort(heads(all()), 'desc'), $nbheads)"