in_tail: fix slice length to reduce memory usage

[Watson1978]

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:
String#slice will allocate redundant heap area internally if an incorrect length is specified.
This PR might reduce the memory usage with in_tail plugin.

• verify code

require 'bundler/inline'
gemfile do
  source 'https://rubygems.org'
  gem 'benchmark-memory'
end

str = "a" * 10_000_000
split_point = 9_999_900

Benchmark.memory do |x|
  x.report("1") {
    str.slice(split_point, str.length)
  }

  x.report("2") {
    str.slice(split_point, str.length - split_point)
  }

  x.report("3") {
    str.slice(split_point..str.length)
  }
end

• result

Calculating -------------------------------------
                   1    10.000M memsize (    10.000M retained)
                         2.000  objects (     1.000  retained)
                         2.000  strings (     1.000  retained)
                   2    40.000  memsize (     0.000  retained)
                         1.000  objects (     0.000  retained)
                         1.000  strings (     0.000  retained)
                   3    80.000  memsize (     0.000  retained)
                         2.000  objects (     0.000  retained)
                         1.000  strings (     0.000  retained)

The all cases will generate same String object. However, 1 case will use 10 MB heap area with above code.

Docs Changes:

Release Note:

[kenhys]

memo:

index: 0.... k.....n-1 (total n)
k = (idx + 1) = slice_position

sliced length: 
(n-1) - k + 1
= n - k
= n - (idx + 1)
= n - slice_position

[daipom]

I'm not going to backport this to LTS (v1.16.x), but if anyone has a request, please let me know.