This is similar to f7dac82ed (Escape separators (colon and equals) to improve
completion, 2019-08-23) except we only escape : and = if they are the result of
file completions. This way we avoid issues with custom completions like dd.
This also means that it won't work for things like __fish_complete_suffix
[*] but that can be fixed later, once we can serialize the DONT_ESCAPE flag.
By moving the escaping step earlier, this causes some unit test changes
which should not result in actual behavior change.
See also #6099
[*]: The new \: and \= does not leak from "complete -C" because that command
unescapes its output -- unless --escape is given.
This is the last remnant of the old percent expansion.
It has the downsides of it, in that it is annoying to combine with
anything:
```fish
echo %self/foo
```
prints "%self/foo", not fish's pid.
We have introduced $fish_pid in 3.0, which is much easier to use -
just like a variable, because it is one.
If you need backwards-compatibility for < 3.0, you can use the
following shim:
```fish
set -q fish_pid
or set -g fish_pid %self
```
So we introduce a feature-flag called "remove-percent-self" to turn it
off.
"%self" will simply not be special, e.g. `echo %self` will print
"%self".
No need to use cfg_attr and have to worry about syncing the preconditions for
the cfg_attr with the preconditions for where `slice_contains_slice()` is used
in the codebase, just mark it as `allow(unused)` with a comment.
