revlog: use an intermediate variable for `data` in `_chunk`

This will help us to dispatch this to the right decompressor.

Differential Revision: https://phab.mercurial-scm.org/D10649

mercurial/revlog.py

@@ -1810,7 +1810,8 b' class revlog(object):'
                 if inline:
                     chunkstart += (rev + 1) * iosize
                 chunklength = length(rev)
-                ladd(decomp(buffer(data, chunkstart - offset, chunklength)))
+                c = buffer(data, chunkstart - offset, chunklength)
+                ladd(decomp(c))
 
         return l