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

rhg: stop shadowing `exit` function This will be useful for the next patch which needs it.

config: add alias from `hg help rhg` to `hg help rust` This will make using `rhg` more user-friendly and features more discoverable.

rhg: add `config.rhg` helptext This will make using `rhg` more user-friendly and features more discoverable.

config: fix indentation of some`share-safe` options This makes the output much more readable.