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
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
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$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
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See https://mercurial-scm.org/ for detailed installation
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