Restore piping support.

Add a ShellManager::pipe method, which pipes a string into the
command. Not quite satisfied with this interface.

src/main.cc

@@ -215,7 +215,8 @@ void do_pipe(Editor& editor, int count)
         editor.buffer().begin_undo_group();
         for (auto& sel : const_cast<const Editor&>(editor).selections())
         {
-            String new_content = ShellManager::instance().eval(cmdline, main_context, {});
+            String new_content = ShellManager::instance().pipe(String(sel.begin(), sel.end()),
+                                                               cmdline, main_context, {});
             editor.buffer().modify(Modification::make_erase(sel.begin(), sel.end()));
             editor.buffer().modify(Modification::make_insert(sel.begin(), new_content));
         }

src/shell_manager.cc

@@ -6,26 +6,37 @@
 
 namespace Kakoune
 {
+String env_var_regex(R"(\$\{kak_([a-z0-9_]+)[^}]*\}|\$kak_([a-z0-9_]+))");
 
 ShellManager::ShellManager()
-   : m_regex(LR"(\$\{kak_([a-z0-9_]+)[^}]*\}|\$kak_([a-z0-9_]+))")
+   : m_regex(env_var_regex.begin(), env_var_regex.end())
 {
 }
 
 String ShellManager::eval(const String& cmdline, const Context& context,
                           const EnvVarMap& env_vars)
+{
+    return pipe("", cmdline, context, env_vars);
+}
+
+String ShellManager::pipe(const String& input,
+                          const String& cmdline, const Context& context,
+                          const EnvVarMap& env_vars)
 {
     int write_pipe[2];
     int read_pipe[2];
 
-    pipe(write_pipe);
+    ::pipe(write_pipe);
-    pipe(read_pipe);
+    ::pipe(read_pipe);
 
     String output;
     if (pid_t pid = fork())
     {
         close(write_pipe[0]);
         close(read_pipe[1]);
+
+        memoryview<char> data = input.data();
+        write(write_pipe[1], data.pointer(), data.size());
         close(write_pipe[1]);
 
         char buffer[1024];

src/shell_manager.hh

@@ -21,6 +21,10 @@ public:
     String eval(const String& cmdline, const Context& context,
                 const EnvVarMap& env_vars);
 
+    String pipe(const String& input,
+                const String& cmdline, const Context& context,
+                const EnvVarMap& env_vars);
+
     void register_env_var(const String& name, EnvVarRetriever retriever);
 
 private: