Refactor parse_hook_folder() to be more robust and testable

Introduction

Config Commands are defined by struct Command and registered with commands_register().
When NeoMutt reads a config file, it calls parse_rc_line() for each line.
parse_rc_line() matches the text to Command.name, then calls Command.parse() to do the actual parsing and storage.

The prototype for the folder-hook Command is:

• folder-hook [ -noregex ] <regex> <command>

Sample valid Commands:

folder-hook "." "set collapse_unread"
folder-hook "=trash" "macro index d <delete-message>"

Problem

parse_hook_folder() parses the folder-hook Command.
It stores the result in the global Hooks, which makes it hard to unit test the function.

Task

Split out the parsing from parse_hook_folder() into a new function that can be tested thoroughly.

Goals

• Simple parsing function for folder-hook
• On success: return a struct Hook
• On error:
  • Return a clear error message
  • Return the location of the error in the string
  • Error return mechanism should be reusable for other Command parsing functions
• Testing:
  • Valid config returns a struct Hook with the correct contents
  • Invalid config returns an error message and the location of the error
  • The tests provide 100% code coverage of the parsing function

Testing

Build:

• ./configure --testing --coverage
• make to build NeoMutt
• make test to build and run the unit tests
• make coverage to perform code coverage tests
  (Results are saved in the coverage directory)