rev-branch-cache: reenable memory mapping of the revision data

Now that we are no longer truncating it, we can mmap it again.

This provide a sizeable speedup on repository with a very large amount of
revision for example for a mozilla-try clone with 5 793 383 revisions, this
provide a speedup of 5ms - 10ms. Since they happens within the "critical" locked
path during push. These miliseconds are important.

In addition, the v3 branchmap format is use the rev-branch-cache more than the
v2 branchmap cache so this will be important.

On smaller repository we consistently see an improvement of one or two percents,
but the gain in absolute time is usually < 10 ms.

#### benchmark.name                                 = hg.command.unbundle
   # benchmark.variants.issue6528                   = disabled
   # benchmark.variants.reuse-external-delta-parent = yes
   # benchmark.variants.revs                        = any-1-extra-rev
   # benchmark.variants.source                      = unbundle
   # benchmark.variants.verbosity                   = quiet

 ### data-env-vars.name = mozilla-try-2024-03-26-zstd-sparse-revlog
  ## bin-env-vars.hg.flavor = default
e51161b12c7e: 3.527923
ebdcfe85b070: 3.468178   (-1.69%, -0.06)
  ## bin-env-vars.hg.flavor = rust
e51161b12c7e: 3.580158
ebdcfe85b070: 3.480564   (-2.78%, -0.10)
 ### data-env-vars.name = mozilla-try-2024-03-26-ds2-pnm
  ## bin-env-vars.hg.flavor = rust
e51161b12c7e: 3.527923
ebdcfe85b070: 3.468178 (-1.69%, -0.06)

mercurial/configitems.toml

@@ -2163,7 +2163,7 experimental = true # experimental as l
 [[items]]
 section = "storage"
 name = "revbranchcache.mmap"
-default = false
+default = true
 
 [[items]]
 section = "storage"