Infer short completion descriptions for commandline flags

Descriptions for commandline flags may not include newlines and should
be rather short for display in a shell. Truncate the description string
of a flag on '\n' or '.' to and add an ellipsis if needed.

src/libutil/args.cc

@@ -29,7 +29,16 @@ void Args::removeFlag(const std::string & longName)
 
 void Completions::add(std::string completion, std::string description)
 {
- assert(description.find('\n') == std::string::npos);
+ // strip whitespace/empty lines from the front of the description
+ description.erase(0, description.find_first_not_of(" \t\n"));
+ // ellipsize overflowing content on the back of the description
+ auto end_index = description.find_first_of(".\n");
+ if (end_index != std::string::npos) {
+ auto needs_ellipsis = end_index != description.size() - 1;
+ description.resize(end_index);
+ if (needs_ellipsis)
+ description.append(" [...]");
+ }
  insert(Completion {
  .completion = completion,
  .description = description