portability: fix build on Solaris-derived systemd Current Illumos and older Solaris require _XOPEN_SOURCE for msg_control. O_DIRECTORY doesn't exist on older systems either, so fallback to O_RDONLY. It's good enough as a repository will require both R and X permission anyway.

mmap: only use mmap to read revlog persistent nodemap if it is safe Cf `is_mmap_safe` docstring.

mmap: fix another instance of reverse mmap logic in persistent nodemap This fix the same kind of issue as 85d96517e650

mmap: only use mmap to read rev-branch-cache data if it is safe Cf `is_mmap_safe` docstring.

mmap: only use mmap to read revlog index if it is safe Cf `is_mmap_safe` docstring.

mmap: add a `is_mmap_safe` method to vfs This will be useful to safeguard mmap usage to void SIGBUS when repositories lives on a NFS drive.

branching: merge stable into default

Added signature for changeset 6454c117c6a4

Added tag 6.8rc0 for changeset 6454c117c6a4

branching: merge default into stable for 6.8rc0