Use fuzzy subsequence completion for options names as well

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.

CHANGELOG.rst

@@ -44,6 +44,7 @@ Interactive improvements
 - Command-specific tab completions may now offer results whose first character is a period. For example, it is now possible to tab-complete ``git add`` for files with leading periods. The default file completions hide these files, unless the token itself has a leading period (:issue:`3707`).
 - The :kbd:`Control-R` history search now uses glob syntax (:issue:`10131`).
 - :kbd:`Alt-E` now passes the cursor position to the external editor also if the editor aliases a supported editor (via ``complete --wraps``).
+- Option completion now uses fuzzy subsequence filtering as well. This means that ``--fb`` may be completed to ``--foobar`` if there is no better match.
 
 New or improved bindings
 ^^^^^^^^^^^^^^^^^^^^^^^^

src/complete.rs

@@ -1435,21 +1435,18 @@ impl<'ctx> Completer<'ctx> {
                 if whole_opt.len() < s.len() {
                     continue;
                 }
-                let r#match = string_prefixes_string(s, &whole_opt);
-                if !r#match {
-                    let match_no_case = string_prefixes_string_case_insensitive(s, &whole_opt);
-                    if !match_no_case {
-                        continue;
-                    }
-                }
+                let anchor_start = !self.flags.fuzzy_match;
+                let Some(r#match) = string_fuzzy_match_string(s, &whole_opt, anchor_start) else {
+                    continue;
+                };
 
                 let mut offset = 0;
                 let mut flags = CompleteFlags::empty();
 
-                if r#match {
-                    offset = s.len();
-                } else {
+                if r#match.requires_full_replacement() {
                     flags = CompleteFlags::REPLACES_TOKEN;
+                } else {
+                    offset = s.len();
                 }
 
                 // does this switch have any known arguments

tests/checks/complete.fish

@@ -593,3 +593,11 @@ complete -C'complete_unescaped_tokens "foo" bar\\ baz (qux) '
 # CHECK: foo
 # CHECK: bar baz
 # CHECK: (qux)
+
+## Fuzzy completion of options
+complete complete_long_option -f -l some-long-option
+complete -C'complete_long_option --slo'
+# CHECK: --some-long-option
+complete complete_long_option -f -o an-old-option
+complete -C'complete_long_option -ao'
+# CHECK: -an-old-option