Remove complete_prefix option and behaviour

Now that we use subsequence based completion almost everywhere,
completing the common prefix does not make sense anymore.

README.asciidoc

@@ -813,8 +813,6 @@ Some options are built in Kakoune, and can be used to control it's behaviour:
    writing a buffer, this is autodetected on load.
  * `BOM` _enum(none|utf8)_: define if the file should be written
    with an unicode byte order mark.
- * `complete_prefix` _bool_: when completing in command line, and multiple
-   candidates exist, enable completion with common prefix.
  * `incsearch` _bool_: execute search as it is typed
  * `aligntab` _bool_: use tabs for alignment command
  * `autoinfo` _flags(command|onkey|normal)_: display automatic information

doc/manpages/options.asciidoc

@@ -66,10 +66,6 @@ Builtin options
 *BOM* 'enum(none|utf8)'::
 	define if the file should be written with an unicode byte order mark
 
-*complete_prefix* 'bool'::
-	when completing in command line, and multiple candidates exist,
-	enable completion with common prefix
-
 *incsearch* 'bool'::
 	execute search as it is typed
 

src/input_handler.cc

@@ -762,51 +762,16 @@ public:
             const bool reverse = (key == Key::BackTab);
             CandidateList& candidates = m_completions.candidates;
             // first try, we need to ask our completer for completions
-            bool updated_completions = false;
             if (candidates.empty())
-            {
                 refresh_completions(CompletionFlags::None);
 
             if (candidates.empty())
                 return;
-                updated_completions = true;
-            }
-            bool did_prefix = false;
-            if (m_current_completion == -1 and
-                context().options()["complete_prefix"].get<bool>())
-            {
-                const String& line = m_line_editor.line();
-                CandidateList& candidates = m_completions.candidates;
-                String prefix = common_prefix(candidates);
-                if (m_completions.end - m_completions.start > prefix.length())
-                    prefix = line.substr(m_completions.start,
-                                         m_completions.end - m_completions.start).str();
 
-                if (not prefix.empty())
-                {
-                    auto it = find(candidates, prefix);
-                    if (it == candidates.end())
-                    {
-                        m_current_completion = candidates.size();
-                        candidates.push_back(prefix);
-                    }
-                    else
-                        m_current_completion = it - candidates.begin();
-
-                    CharCount start = line.char_count_to(m_completions.start);
-                    // When we just updated completions, select the common
-                    // prefix even if it was the currently entered text.
-                    did_prefix = updated_completions or
-                                 prefix != line.substr(start, m_line_editor.cursor_pos() - start);
-                }
-            }
-            if (not did_prefix)
-            {
             if (not reverse and ++m_current_completion >= candidates.size())
                 m_current_completion = 0;
             else if (reverse and --m_current_completion < 0)
                 m_current_completion = candidates.size()-1;
-            }
 
             const String& completion = candidates[m_current_completion];
             if (context().has_client())

src/main.cc

@@ -206,9 +206,6 @@ void register_options()
     reg.declare_option("eolformat", "end of line format: crlf or lf", EolFormat::Lf);
     reg.declare_option("BOM", "insert a byte order mark when writing buffer (none or utf8)",
                        ByteOrderMark::None);
-    reg.declare_option("complete_prefix",
-                       "complete up to common prefix in tab completion",
-                       true);
     reg.declare_option("incsearch",
                        "incrementaly apply search/select/split regex",
                        true);