rhg: add a config option to fall back immediately This is useful for debugging the behavior of the "default" `hg` in tests without having to manually substitute the fallback path.

rust-status: query fs traversal metadata lazily Currently, any time the status algorithm needs to read a directory from the filesystem (because the stat-only optimization is not available), it also stats each directory entry eagerly. Stat'ing the entries is only needed in a few cases (like when checking the mtime of a directory for caching): this patch creates a wrapper struct `DirEntry` that only stats the directory entry it represents when needed. Excerpt of an `strace` before this change on Mozilla Central: ``` openat(AT_FDCWD, ".", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_DIRECTORY) = 3 newfstatat(3, "", {st_mode=S_IFDIR|0755, st_size=3540, ...}, AT_EMPTY_PATH) = 0 getdents64(3, 0x55dc970bd440 /* 139 entries */, 32768) = 5072 statx(3, ".hg", AT_STATX_SYNC_AS_STAT|AT_SYMLINK_NOFOLLOW, STATX_ALL, {stx_mask=STATX_ALL|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFDIR|0755, stx_size=772, ...}) = 0 [... 135 other successful `statx` calls] getdents64(3, 0x55dc970bd440 /* 0 entries */, 32768) = 0 close(3) = 0 ``` After this change: ``` openat(AT_FDCWD, ".", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_DIRECTORY) = 3 newfstatat(3, "", {st_mode=S_IFDIR|0755, st_size=3540, ...}, AT_EMPTY_PATH) = 0 getdents64(3, 0x561567c10190 /* 139 entries */, 32768) = 5072 getdents64(3, 0x561567c10190 /* 0 entries */, 32768) = 0 close(3) = 0 ```

rust-status: make `DirEntry` attributes clearer