mmapindex: set default to 1MB

mmapping index is more efficient if we only need a small part of it.

The 1MB value has been picked arbitrarily, a lower value might be better.

On a large repository with a 60MB index, we see the following performance
gain:

hg perfindex
before: ! wall 0.032023 comb 0.040000 user 0.000000 sys 0.040000 (best of 100)
after:  ! wall 0.000196 comb 0.000000 user 0.000000 sys 0.000000 (best of 1060)

The speed boost benefit all cases, including the one where the full index
needs to be parsed.

hg perfindex --rev 0
before: ! wall 0.040673 comb 0.030000 user 0.000000 sys 0.030000 (best of 100)
after   ! wall 0.010713 comb 0.020000 user 0.010000 sys 0.010000 (best of 212)

This gain reflect in higher level operation:

hg perfbookmarks --clear-revlogs
before: ! wall 0.161339 comb 0.160000 user 0.130000 sys 0.030000 (best of 56)
after:  ! wall 0.123228 comb 0.120000 user 0.120000 sys 0.000000 (best of 68)

mercurial/configitems.py

@@ -976,7 +976,7 b" coreconfigitem('push', 'pushvars.server'"
     default=False,
 )
 coreconfigitem('storage', 'mmap-threshold',
-    default=None,
+    default='1MB',
     alias=[('experimental', 'mmapindexthreshold')],
 )
 coreconfigitem('storage', 'new-repo-backend',