Initial support for history search in command-line mode

[LEI]

This is an attempt to implement "partial history search" allowing :a<Up> to show the previous entry starting with "a".

Added search for <Up>/<Down> and <C-p>/<C-n> and changed their behavior, even with no search term:

• :<Up><Down> now goes back to the initial empty state
• and prevent <C-n> or <Down> from cycling back to the first history entry.

The prompt value at the end of the existing history_completion test was "asdf" before but now <tab> seems to have an effect, I am guessing this test should only verify the absence of errors?

Maybe this could apply to "/" and "?" as well.

[pascalkuthe]

It looks like you are doing a simple prefix match here. Using fuzzy matching would be preferable I think we use it for all completions (both in the editor and command mode)

[LEI]

How would fuzzy matching work more precisely? I would not expect :b<up> to match abc for example.

I think this is the default in Vim, and a similar keybind is possible with readline:

   history-search-backward
          Search backward through the history for the string of
          characters between the start of the current line and the
          current cursor position (the point).  The search string
          must match at the beginning of a history line.  This is a
          non-incremental search.

[the-mikedavis]

Duplicate of #2796

[LEI]

Thanks, somehow missed this! I guess the implementation is better and it applies to / & ?.
One difference being C-p and C-n both cycle instead of stopping at the first/last entry (this may be better too, I'll give it a try).

Would it help if I try to add a few integration tests to this PR (requiring visibility changes)?