tests: fix hidden repeatability problem with age filter test

This test was corrupting later cases in mysterious ways because the
hash of the changeset was changing every day at midnight. This made it
hard to figure out why tests were consistently breaking after they
were consistently working.

tests/test-command-template.t

@@ -1938,6 +1938,8 b' Formatnode filter works:'
 
 Age filter:
 
+  $ hg init unstable-hash
+  $ cd unstable-hash
   $ hg log --template '{date|age}\n' > /dev/null || exit 1
 
   >>> from datetime import datetime, timedelta
@@ -1951,6 +1953,15 b' Age filter:'
   $ hg log -l1 --template '{date|age}\n'
   7 years from now
 
+  $ cd ..
+  $ rm -rf unstable-hash
+
+Add a dummy commit to make up for the instability of the above:
+
+  $ echo a > a
+  $ hg add a
+  $ hg ci -m future
+
 Count filter:
 
   $ hg log -l1 --template '{node|count} {node|short|count}\n'