This tries to open the given file to use as stdin, and if it fails,
for any reason, it uses /dev/null instead.
This is useful in cases where we would otherwise do either of these:
```fish
test -r /path/to/file
and string match foo < /path/to/file
cat /path/to/file 2>/dev/null | string match foo
```
This both makes it nicer and shorter, *and* helps with TOCTTOU - what if the file is removed/changed after the check?
The reason for reading /dev/null instead of a closed fd is that a closed fd will often cause an error.
In case opening /dev/null fails, it still skips the command.
That's really a last resort for when the operating system
has turned out to be a platypus and not a unix.
Fixes#4865
(cherry picked from commit df8b9b7095)
File names that have lots of spaces look quite ugly when inserted as
completions because every space will have a backslash.
Add an initial heuristic to decide when to use quotes instead of
backslash escapes.
Quote when
1. it's not an autosuggestion
2. we replace the token or insert a fresh one
3. we will add a space at the end
In future we could relax some of these requirements.
Requirement 2 means we don't quote when appending to an existing token.
Need to find a natural behavior here.
Re 3, if the completion adds no space, users will probably want to add more
characters, which looks a bit weird if the token has a trailing quote.
We could relax this requirement for directory completions, so «ls so»
completes to «ls 'some dir with spaces'/».
Closes#5433
