revlog: early return in _slicechunk when span is already small enough

If the full span is smaller than the minimum gap size we'll consider, we know we
won't do any slicing and we can return earlier.

mercurial/revlog.py

@@ -227,6 +227,10 b' def _slicechunk(revlog, revs):'
     endbyte = start(revs[-1]) + length(revs[-1])
     readdata = deltachainspan = endbyte - startbyte
 
+    if deltachainspan < revlog._srmingapsize:
+        yield revs
+        return
+
     chainpayload = sum(length(r) for r in revs)
 
     if deltachainspan: