util: limit output chunk size in zlib decompression

This is essentially a port of 65bd4b8e48bd, which was inadvertently
dropped by 8cd7d0fefd30.

mercurial/util.py

@@ -3124,7 +3124,10 b' class _zlibengine(compressionengine):'
         def gen():
             d = zlib.decompressobj()
             for chunk in filechunkiter(fh):
-                yield d.decompress(chunk)
+                while chunk:
+                    # Limit output size to limit memory.
+                    yield d.decompress(chunk, 2 ** 18)
+                    chunk = d.unconsumed_tail
 
         return chunkbuffer(gen())