revlog: use an explicit config option to enable mmap usage for index
We replace the `experimental.mmapindexthreshold` with two options:
The `storage.revlog.mmap.index` is a boolean option to enable or disable the
feature. The `storage.revlog.mmap.index:size-threshold` is a bytes option that
control when we will be using mmap instead of plain reading.
https://bz.mercurial-scm.org/612
$ hg init repo
$ cd repo
$ mkdir src
$ echo a > src/a.c
$ hg ci -Ama
adding src/a.c
$ hg mv src source
moving src/a.c to source/a.c
$ hg ci -Ammove
$ hg co -C 0
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ echo new > src/a.c
$ echo compiled > src/a.o
$ hg ci -mupdate
created new head
$ hg status
? src/a.o
$ hg merge
merging src/a.c and source/a.c to source/a.c
0 files updated, 1 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
$ hg status
M source/a.c
R src/a.c
? src/a.o