Fix incremental search cancel bug

[tompng]

Readline restores line, cursor position and history index with "\C-g"(cancel incremental search). Reline should also restore them.
Moving history is difficult. We should use utility method move_history.

Fix incremental search cancel behavior

Input

123\n
456\n
UP
LFET
\C-r
12
\C-g

Expected (Readline, this branch)

irb(main):001> 123
=> 123
irb(main):002> 456
=> 456
irb(main):003> 45|6

Actual

irb(main):001> 123
=> 123
irb(main):002> 456
=> 456
irb(main):003> |123

Fix irb crash

input

1\n
\C-r
1
TAB
\C-r
1
TAB

bug

reline/line_editor.rb:1712:in 'block in Reline::LineEditor#incremental_search_history': undefined method 'split' for nil (NoMethodError)
@buffer_of_lines = line.split("\n")

[ima1zumi]

Thank you for your refactoring and bugfix!