firstmergecache: fix a typo that prevented cache clearing in repo.destroyed()

--HG--
branch : stable
8 jobs for topic/stable/caches in 4 minutes and 52 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed checks-py2 #178959

00:03:18

passed checks-py3 #178960

00:01:07

passed doc #178965

00:00:45

manual pytype #178958
allowed to fail manual
passed tests-py2-cext #178961

00:04:35

passed tests-py2-pure #178962

00:04:08

passed tests-py3-cext #178963

00:03:25

passed tests-py3-pure #178964

00:04:51