revlog: remove unnecessary cache validation in _chunks

Previously, we likely called _chunkraw() multiple times in order to
ensure it didn't change out from under us. I'm pretty certain this code
had its origins in the days where we attempted to have thread safety of
localrepository and thus revlog instances.

revlog instances are already not thread safe for writing. And, as of
Mercurial 3.6, hgweb uses a separate localrepository instance per
request, so there should only be a single thread reading a revlog at
a time. We more or less decided that attempting to make classes like
revlog thread safe is a lost cause.

So, this patch removes thread safety from _chunks. As a result, we make
one less call into _chunkraw() when the initial read isn't serviced
by the cache. This translates to savings of 4 function calls overall
and possibly prevents the creation of an additional buffer view into the
cache. I doubt this translates into any real world performance wins
because decompression will almost certainly dwarf time spent in
_chunks(). But it does make the code simpler, so it is an improvement.

mercurial/revlog.py

@@ -1122,15 +1122,8 b' class revlog(object):'
         l = []
         ladd = l.append
 
-        # preload the cache
         try:
-            while True:
-                # ensure that the cache doesn't change out from under us
-                _cache = self._chunkcache
-                self._chunkraw(revs[0], revs[-1], df=df)[1]
-                if _cache == self._chunkcache:
-                    break
-            offset, data = _cache
+            offset, data = self._chunkraw(revs[0], revs[-1], df=df)
         except OverflowError:
             # issue4215 - we can't cache a run of chunks greater than
             # 2G on Windows