Rename save_command() to save_input_to_history()

In cmdline.c.  Because it also saves search patterns.
Update save_input_to_history() function

* More accurate head comment.
* Better name of the second parameter.
* Less nesting in function body.
Move prompt history save to save_input_to_history

This way cancelled prompt input will be automatically saved into prompt
history.
Document storing cancelled prompt input in history
Merge branch 'save-cancelled-prompt-input-in-history'

Save cancelled prompt input in history, so it can be recalled later.

That is handy if you need to change directory or look into file before
continuing line editing.