Accept "cd dir/" again instead of using a subdirectory

Commit 69053d962 (Use menu behavior when completing change-directory,
2022-07-19) made ":cd dir/" actually run ":cd dir/first-subdir",
which can be surprising.
This is usually irrelevant because you rarely type the trailing slash.
However it does happen after correcting an error with `<backspace>`
and friends. For for example,

	:cd d<tab>/f<backspace>

results in

	:cd dir/

We should probably fix user expectations here. Do this by adding "dir/"
as valid completion.  This requires us to allow empty candidates in
"RankedMatch" but there's no harm in that. This means we need to
filter out empty completions from shell-script-candidates elsewhere.

Alternative fix: we could revert 69053d962. This would remove the
convenient menu behavior but that wouldn't be a huge deal.

Fixes #4775

src/commands.cc

@@ -241,7 +241,8 @@ struct ShellScriptCompleter
                                                       ShellManager::Flags::WaitForStdout,
                                                       shell_context).first;
         CandidateList candidates;
-        for (auto&& candidate : output | split<StringView>('\n'))
+        for (auto&& candidate : output | split<StringView>('\n')
+                                       | filter([](auto s) { return not s.empty(); }))
             candidates.push_back(candidate.str());
 
         return {0_byte, pos_in_token, std::move(candidates), m_flags};
@@ -274,7 +275,8 @@ struct ShellCandidatesCompleter
                                                           ShellManager::Flags::WaitForStdout,
                                                           shell_context).first;
             m_candidates.clear();
-            for (auto c : output | split<StringView>('\n'))
+            for (auto c : output | split<StringView>('\n')
+                                 | filter([](auto s) { return not s.empty(); }))
                 m_candidates.emplace_back(c.str(), used_letters(c));
             m_token = token_to_complete;
         }

src/file.cc

@@ -518,6 +518,14 @@ CandidateList complete_filename(StringView prefix, const Regex& ignored_regex,
         if (RankedMatch match{file, fileprefix})
             matches.push_back(match);
     }
+    // Hack: when completing directories, also echo back the query if it
+    // is a valid directory. This enables menu completion to select the
+    // directory instead of a child.
+    if (only_dirs and not dirname.empty() and dirname.back() == '/' and fileprefix.empty()
+        and /* exists on disk */ not files.empty())
+    {
+        matches.push_back(RankedMatch{fileprefix, fileprefix});
+    }
     std::sort(matches.begin(), matches.end());
     const bool expand = (flags & FilenameFlags::Expand);
     return candidates(matches, expand ? parsed_dirname : dirname);

src/ranked_match.cc

@@ -112,12 +112,13 @@ static Optional<SubseqRes> subsequence_match_smart_case(StringView str, StringVi
 template<typename TestFunc>
 RankedMatch::RankedMatch(StringView candidate, StringView query, TestFunc func)
 {
-    if (candidate.empty() or query.length() > candidate.length())
+    if (query.length() > candidate.length())
         return;
 
     if (query.empty())
     {
         m_candidate = candidate;
+        m_matches = true;
         return;
     }
 
@@ -129,6 +130,7 @@ RankedMatch::RankedMatch(StringView candidate, StringView query, TestFunc func)
         return;
 
     m_candidate = candidate;
+    m_matches = true;
     m_max_index = res->max_index;
 
     if (res->single_word)

src/ranked_match.hh

@@ -27,7 +27,7 @@ struct RankedMatch
     bool operator<(const RankedMatch& other) const;
     bool operator==(const RankedMatch& other) const { return m_candidate == other.m_candidate; }
 
-    explicit operator bool() const { return not m_candidate.empty(); }
+    explicit operator bool() const { return m_matches; }
 
 private:
     template<typename TestFunc>
@@ -48,6 +48,7 @@ private:
     friend constexpr bool with_bit_ops(Meta::Type<Flags>) { return true; }
 
     StringView m_candidate{};
+    bool m_matches = false;
     Flags m_flags = Flags::None;
     int m_word_boundary_match_count = 0;
     int m_max_index = 0;