vfs: make the default opener mode binary The default was already binary for `abstractvfs`, and the `vfs` implementation adds binary mode if the caller didn't supply it. Therefore, it should be safe for all vfs objects (and I don't think we want text reads anyway).

typing: add basic type hints to vfs.py Again, there's a lot more that could be done, but this sticks to the obviously correct stuff that is related to primitives or `vfs` objects. Hopefully this helps smoke out more path related bytes vs str issues in TortoiseHg. PyCharm seems smart enough to apply hints from annotated superclass functions, but pytype isn't (according to the *.pyi file generated), so those are annotated too. There was some discussion about changing the default path arg from `None` to `b''` in order to avoid the more verbose `Optional` declarations. This would be more in line with `os.path.join()` (which rejects `None`, but ignores empty strings), and still not change the behavior for callers still passing `None` (because the check is `if path` instead of an explicit check for `None`). But I didn't want to hold this up while discussing that, so this documents what _is_.

util: implement `writelines()` on atomictempfile With typehints on the vfs objects, pytype will flag this: FAILED: /mnt/c/Users/Matt/hg/.pytype/pyi/mercurial/patch.pyi /usr/bin/python3.8 -m pytype.single --imports_info /mnt/c/Users/Matt/hg/.pytype/imports/mercurial.patch.imports --module-name mercurial.patch -V 3.7 -o /mnt/c/Users/Matt/hg/.pytype/pyi/mercurial/patch.pyi --analyze-annotated --nofail --quick /mnt/c/Users/Matt/hg/mercurial/patch.py File "/mnt/c/Users/Matt/hg/mercurial/patch.py", line 535, in writerej: No attribute 'writelines' on mercurial.util.atomictempfile [attribute-error] In Union[ mercurial.util.atomictempfile, mercurial.vfs.checkambigatclosing, mercurial.vfs.delayclosedfile, mercurial.windows.fdproxy, mercurial.windows.mixedfilemodewrapper ] It's not a real problem there (atomictempfile is only created by passing different args), but it's reasonable for this to implement the function and behave like a normal file. There are other functions missing that can be added if/when needed.

typing: add basic type hints to localrepo.py There's a lot more that could be done, but this sticks to the obviously correct stuff that is either related to existing imports or primitives. Hopefully this helps smoke out more path related bytes vs str issues in TortoiseHg. I'm avoiding the interfaces for now, because they seem to confuse pytype and/or PyCharm. It might be worth typing the return of `makelocalrepository` to `localrepository`, but that leaks an implementation detail, so that can be revisited later.

check-code: drop the check for whitespace around named parameters This check flags py3 annotations of named parameters, because `black` adds spaces around the assignment in this case. Since the chosen formatter has opinions (and pylint also wants the space in the case of annotations), drop the check so we can use py3 annotations.

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

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.