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.

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