revbranchcache: disable mmap access by default

The revbranchcache can be truncated (if some part of it is detected as invalid).
Using mmap on file we truncate is not an option at access to truncated part
would result in a SIGBUS signal.

So we disable the mmap by default until we fix this issue.

mercurial/configitems.toml

@@ -2147,7 +2147,7 b' experimental = true # experimental as l'
 [[items]]
 section = "storage"
 name = "revbranchcache.mmap"
-default = true
+default = false
 
 [[items]]
 section = "storage"