[PATCH] Unset environment variable 'P' in test-revtest2.t

Gentoo ebuilds define and export the variable P, which is set to the
package name ('mercurial-4.5.2' in this case). The test-revset2.t test
expects that no environment variable P is set and fails if it set to a
non-existing revision (such as 'mercurial').

This is known as Gentoo bug #637240 [0].

[0] https://bugs.gentoo.org/637240
---
 tests/test-revset2.t | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tests/test-revset2.t b/tests/test-revset2.t
index b5a8ba8..419dd09 100644
--- a/tests/test-revset2.t
+++ b/tests/test-revset2.t
@@ -1627,6 +1627,7 @@ Test repo.anyrevs with customized revset overrides
   > printprevset = $TESTTMP/printprevset.py
   > EOF

+  $ unset P
   $ hg --config revsetalias.P=1 printprevset
   P=[1]
   $ P=3 hg --config revsetalias.P=2 printprevset