tests: unset environment variable P in test-revset2.t (issue6109)

The test tests/test-revset2.t fails the test case

"Test repo.anyrevs with customized revset overrides" (line 1609)

if the environment variable P is set. The test implicitly expects that the
environment, in which it is started, does not export the variable 'P'.

To solve this issue, unset 'P' right before the test commands are run.

Differential Revision: https://phab.mercurial-scm.org/D6195

tests/test-revset2.t

@@ -1627,6 +1627,7 b' Test repo.anyrevs with customized revset'
   > printprevset = $TESTTMP/printprevset.py
   > EOF
 
+  $ unset P
   $ hg --config revsetalias.P=1 printprevset
   P=[1]
   $ P=3 hg --config revsetalias.P=2 printprevset