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.
Multiline search strings are weirdly broken (inserting control characters
in the command line) and probably not very useful anyway.
On the other hand I often want to compose a multi-line command
from single-line commands I ran previously.
Let's support this case by limiting the initial search string to the current
line; and replace only that line.
Alternatively this could operate on jobs (that is, replace a surrounding
"foo | bar") instead of using line boundaries.
Commit b768b9d3f (Use fuzzy subsequence completion for options names as well,
2024-01-27) allowed completing "oa" to "--foobar", which is a false positive,
especially because it hides other valid completions of non-option arguments.
Let's at least require a leading dash again before completing option names.
Version 2.1.0 introduced subsequence matching for completions but as the
changelog entry mentions, "This feature [...] is not yet implemented for
options (like ``--foobar``)". Add it. Seems like a strict improvement,
pretty much.
