sparse-read: target density of 50% instead of 25%

The target density value is wrong. The default target chain span is
4*text-length. However, the target max chain payload is 2*text-length. So
default target density should be 50% (2/4) not 25% (1/4).

mercurial/configitems.py

@@ -594,7 +594,7 b" coreconfigitem('experimental', 'sparse-r"
     default=False,
 )
 coreconfigitem('experimental', 'sparse-read.density-threshold',
-    default=0.25,
+    default=0.50,
 )
 coreconfigitem('experimental', 'sparse-read.min-gap-size',
     default='256K',

mercurial/revlog.py

@@ -625,7 +625,7 b' class revlog(object):'
         self._compengine = 'zlib'
         self._maxdeltachainspan = -1
         self._withsparseread = False
-        self._srdensitythreshold = 0.25
+        self._srdensitythreshold = 0.50
         self._srmingapsize = 262144
 
         mmapindexthreshold = None