Enable case-insensitive substring fuzzy matching

Adds a new match mode for `string_fuzzy_match_t` that matches against a
case-insensitive subsequence within a string, e.g. `LL` now (partially)
matches against `hello`. This is implemented as a separate mode, given a
lower priority of match than a same-case match (when present).

Note that `fuzzy_match_subsequence_insertions_only` has purposely not
been extended with a case-insensitive version as that would be a)
unlikely to match often, and b) adding a second inefficient fuzzy search
to something that's queried a lot. Perhaps `subsequence_insertions_only`
can simply be changed to be a case-insensitive comparison in the future?

Closes #1196. Affects #3978.

src/common.cpp

@@ -1835,6 +1835,13 @@ string_fuzzy_match_t string_fuzzy_match_string(const wcstring &string,
         assert(match_against.size() >= string.size());
         result.match_distance_first = match_against.size() - string.size();
         result.match_distance_second = location;  // prefer earlier matches
+    } else if (limit_type >= fuzzy_match_substring &&
+               (location = ifind(match_against, string)) != wcstring::npos) {
+        // A case-insensitive version of the string is in the match against.
+        result.type = fuzzy_match_substring_case_insensitive;
+        assert(match_against.size() >= string.size());
+        result.match_distance_first = match_against.size() - string.size();
+        result.match_distance_second = location;  // prefer earlier matches
     } else if (limit_type >= fuzzy_match_subsequence_insertions_only &&
                subsequence_in_string(string, match_against)) {
         result.type = fuzzy_match_subsequence_insertions_only;

src/common.h

@@ -399,6 +399,9 @@ enum fuzzy_match_type_t {
     // We match a substring of the string: OOBA matches FOOBAR.
     fuzzy_match_substring,
 
+    // We match a substring of the string: ooBA matches FOOBAR.
+    fuzzy_match_substring_case_insensitive,
+
     // A subsequence match with insertions only: FBR matches FOOBAR.
     fuzzy_match_subsequence_insertions_only,