revlog: optimize _chunkraw when startrev==endrev

In many cases, _chunkraw() is called with startrev==endrev. When
this is true, we can avoid an extra index lookup and some other
minor operations.

On the mozilla-unified repo, `hg perfrevlogchunks -c` says this
has the following impact:

! read w/ reused fd
! wall 0.371846 comb 0.370000 user 0.350000 sys 0.020000 (best of 27)
! wall 0.337930 comb 0.330000 user 0.300000 sys 0.030000 (best of 30)

! read batch w/ reused fd
! wall 0.014952 comb 0.020000 user 0.000000 sys 0.020000 (best of 197)
! wall 0.014866 comb 0.010000 user 0.000000 sys 0.010000 (best of 196)

So, we've gone from ~25x slower than batch to ~22.5x slower.

At this point, there's probably not much else we can do except
implement an optimized function in the index itself, including in C.

mercurial/revlog.py

@@ -1113,9 +1113,12 b' class revlog(object):'
         # (functions are expensive).
         index = self.index
         istart = index[startrev]
-        iend = index[endrev]
         start = int(istart[0] >> 16)
-        end = int(iend[0] >> 16) + iend[1]
+        if startrev == endrev:
+            end = start + istart[1]
+        else:
+            iend = index[endrev]
+            end = int(iend[0] >> 16) + iend[1]
 
         if self._inline:
             start += (startrev + 1) * self._io.size