Call load_command_list() only when it is needed

This avoids list_commands_in_dir() being called when not needed which is
quite slow due to file I/O in order to list matching files in a directory.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Speed up is_git_command() by checking early for internal commands

This avoids the quite slow load_command_list() / list_commands_in_dir() to
be called for help on internal commands.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>