mmapindex: move the 'mmapindexthreshold' option out of experimental

The option is useful and should be advertised more. We move it out of
experimental as a first step. The `storage` section is selected as this is
related to how the storage is accessed. A new 'performance' section might be
more appropriate.

We move from 'mmapindexthreshold` to `mmap-threshold` as non-index item are
also suitable for mmap (eg: the rev-branch-cache).

If relevant, we can introduce sub-option `mmap-threshold.revlog-index` later.

mercurial/configitems.py

@@ -544,9 +544,6 b" coreconfigitem('experimental', 'maxdelta"
 coreconfigitem('experimental', 'mergetempdirprefix',
     default=None,
 )
-coreconfigitem('experimental', 'mmapindexthreshold',
-    default=None,
-)
 coreconfigitem('experimental', 'narrow',
     default=False,
 )
@@ -978,6 +975,10 b" coreconfigitem('progress', 'width',"
 coreconfigitem('push', 'pushvars.server',
     default=False,
 )
+coreconfigitem('storage', 'mmap-threshold',
+    default=None,
+    alias=[('experimental', 'mmapindexthreshold')],
+)
 coreconfigitem('storage', 'new-repo-backend',
     default='revlogv1',
 )

mercurial/localrepo.py

@@ -751,8 +751,7 b' def resolverevlogstorevfsoptions(ui, req'
     if 0 <= chainspan:
         options[b'maxdeltachainspan'] = chainspan
 
-    mmapindexthreshold = ui.configbytes(b'experimental',
-                                        b'mmapindexthreshold')
+    mmapindexthreshold = ui.configbytes(b'storage', b'mmap-threshold')
     if mmapindexthreshold is not None:
         options[b'mmapindexthreshold'] = mmapindexthreshold