Merge branch 'ag/rebase-i-in-c'

Rewrite of the remaining "rebase -i" machinery in C.

* ag/rebase-i-in-c:
  rebase -i: move rebase--helper modes to rebase--interactive
  rebase -i: remove git-rebase--interactive.sh
  rebase--interactive2: rewrite the submodes of interactive rebase in C
  rebase -i: implement the main part of interactive rebase as a builtin
  rebase -i: rewrite init_basic_state() in C
  rebase -i: rewrite write_basic_state() in C
  rebase -i: rewrite the rest of init_revisions_and_shortrevisions() in C
  rebase -i: implement the logic to initialize $revisions in C
  rebase -i: remove unused modes and functions
  rebase -i: rewrite complete_action() in C
  t3404: todo list with commented-out commands only aborts
  sequencer: change the way skip_unnecessary_picks() returns its result
  sequencer: refactor append_todo_help() to write its message to a buffer
  rebase -i: rewrite checkout_onto() in C
  rebase -i: rewrite setup_reflog_action() in C
  sequencer: add a new function to silence a command, except if it fails
  rebase -i: rewrite the edit-todo functionality in C
  editor: add a function to launch the sequence editor
  rebase -i: rewrite append_todo_help() in C
  sequencer: make three functions and an enum from sequencer.c public

strbuf.h

@@ -582,10 +582,12 @@ void strbuf_add_unique_abbrev(struct strbuf *sb,
  * run in. If the buffer is NULL the editor is launched as usual but the
  * file's contents are not read into the buffer upon completion.
  */
-int launch_editor(const char *path,
-		  struct strbuf *buffer,
+int launch_editor(const char *path, struct strbuf *buffer,
 		  const char *const *env);
 
+int launch_sequence_editor(const char *path, struct strbuf *buffer,
+			   const char *const *env);
+
 void strbuf_add_lines(struct strbuf *sb,
 		      const char *prefix,
 		      const char *buf,