revlog: add attribute on revlogs that specifies its kind

The sidedata logic needs to check whether the revlog it's working on is a
changelog, a manifest or a filelog.
Furthermore, future versions of the revlog format will most likely see a split
between the three types (i.e. they will store different information), so having
this will be useful for other future endeavors as well.

Differential Revision: https://phab.mercurial-scm.org/D10151

mercurial/changelog.py

@@ -428,6 +428,7 b' class changelog(revlog.revlog):'
         self._filteredrevs = frozenset()
         self._filteredrevs_hashcache = {}
         self._copiesstorage = opener.options.get(b'copies-storage')
+        self.revlog_kind = b'changelog'
 
     @property
     def filteredrevs(self):

mercurial/filelog.py

@@ -32,6 +32,7 b' class filelog(object):'
         # Full name of the user visible file, relative to the repository root.
         # Used by LFS.
         self._revlog.filename = path
+        self._revlog.revlog_kind = b'filelog'
 
     def __len__(self):
         return len(self._revlog)

mercurial/manifest.py

@@ -1610,6 +1610,7 b' class manifestrevlog(object):'
         self.index = self._revlog.index
         self.version = self._revlog.version
         self._generaldelta = self._revlog._generaldelta
+        self._revlog.revlog_kind = b'manifest'
 
     def _setupmanifestcachehooks(self, repo):
         """Persist the manifestfulltextcache on lock release"""