util: make `mmapread()` work on Windows again

522b4d729e89 started referencing `mmap.MAP_PRIVATE`, but that's not available on
Windows, so `hg version` worked, but `make local` did not.  That commit also
started calling the constructor with the fine-grained `flags` and `prot` args,
but those aren't available on Windows either[1] (though the backing C code
doesn't seem conditionalized to disallow usage of them).

I assume the change away from from the `access` arg was to provide the same
options, plus `MAP_POPULATE`.  Looking at the source code[2], they're not quite
the same- `ACCESS_READ` is equivalent to `flags = MAP_SHARED` and `prot = PROT_READ`.
`MAP_PRIVATE` is only used with `ACCESS_COPY`, which allows read and write.
Therefore, we can't quite get the same baseline flags on Windows, but this was
the status quo ante and `MAP_POPULATE` is a Linux thing, so presumably it works.

I realize that typically the OS differences are abstracted into the platform
modules, but I'm leaving it here so that it is obvious what the differences are
between the platforms.

[1] https://docs.python.org/3/library/mmap.html#mmap.mmap
[2] https://github.com/python/cpython/blob/5e0abb47886bc665eefdcc19fde985f803e49d4c/Modules/mmapmodule.c#L1539

mercurial/util.py

@@ -485,15 +485,24 def mmapread(fp, size=None, pre_populate
     elif size is None:
         size = 0
     fd = getattr(fp, 'fileno', lambda: fp)()
+
+    if pycompat.iswindows:
+        _mmap = lambda fd, size: mmap.mmap(fd, size, access=mmap.ACCESS_READ)
+    else:
     flags = mmap.MAP_PRIVATE
     bg_populate = hasattr(osutil, "background_mmap_populate")
+
     if pre_populate and not bg_populate:
         flags |= getattr(mmap, 'MAP_POPULATE', 0)
-    try:
+
+        def _mmap(fd, size) -> mmap.mmap:
         m = mmap.mmap(fd, size, flags=flags, prot=mmap.PROT_READ)
         if pre_populate and bg_populate:
             osutil.background_mmap_populate(m)
         return m
+
+    try:
+        return _mmap(fd, size)
     except ValueError:
         # Empty files cannot be mmapped, but mmapread should still work.  Check
         # if the file is empty, and if so, return an empty buffer.