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 #178959
checks-py2

00:03:18

passed #178960
checks-py3

00:01:07

passed #178965
doc

00:00:45

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

00:04:35

passed #178962
tests-py2-pure

00:04:08

passed #178963
tests-py3-cext

00:03:25

passed #178964
tests-py3-pure

00:04:51