manifest: set appropriate cache entry when clearing _dirlogcache()
manifestrevlog applies to any manifest, not just the root manifest.
Resetting the cache and populating its root entry with self is not
correct when the instance does not refer to the root manifest.
This has no test fallout. So I'm guessing we only ever call
clearcaches() on the root manifest. Or we have no test coverage that
are impacted by a bad cache on a non-root manifestrevlog.
Differential Revision: https://phab.mercurial-scm.org/D4275
#require execbit
$ hg init repo
$ cd repo
$ echo foo > foo
$ chmod 644 foo
$ hg ci -qAm '644'
$ chmod 755 foo
$ hg ci -qAm '755'
reverting to rev 0
$ hg revert -a -r 0
reverting foo
$ hg st
M foo
$ hg diff --git
diff --git a/foo b/foo
old mode 100755
new mode 100644
$ cd ..