fix: use common lineHash to share indice between text1 and text2

[gkampitakis]

Use common cache of line contents between two texts in DiffLinesToChars to get line diffs correctly. In some cases, line diffs cannot be retrieved correctly in the standard way (https://github.com/google/diff-match-patch/wiki/Line-or-Word-Diffs#line-mode). In the below case, we failed to get line diffs correctly before this fix.

package main

import (
	"fmt"

	"github.com/sergi/go-diff/diffmatchpatch"
)

const (
	text1 = `hoge:
  step11:
  - arrayitem1
  - arrayitem2
  step12:
    step21: hoge
    step22: -93
fuga: flatitem
`
	text2 = `hoge:
  step11:
  - arrayitem4
  - arrayitem2
  - arrayitem3
  step12:
    step21: hoge
    step22: -92
fuga: flatitem
`
)

func main() {
	dmp := diffmatchpatch.New()
	a, b, c := dmp.DiffLinesToChars(text1, text2)
	diffs := dmp.DiffMain(a, b, false)
	diffs = dmp.DiffCharsToLines(diffs, c)
	// diffs = dmp.DiffCleanupSemantic(diffs)
	fmt.Println(diffs)
}

[{Insert hoge:
  step11:
hoge:
} {Equal hoge:
} {Insert hoge:
} {Equal   step11:
} {Insert hoge:
} {Equal   - arrayitem1
} {Insert hoge:
} {Equal   - arrayitem2
} {Insert hoge:
} {Equal   step12:
} {Insert hoge:
} {Equal     step21: hoge
} {Insert hoge:
} {Equal     step22: -93
} {Delete fuga: flatitem
}]

Note: This fix corresponds to a javascript implementation.
(ref: https://github.com/google/diff-match-patch/blob/62f2e689f498f9c92dbc588c58750addec9b1654/javascript/diff_match_patch_uncompressed.js#L466)