utils: diff, skip useless rune->string conversion

According to library documentation :
https://github.com/sergi/go-diff/blob/master/diffmatchpatch/diff.go#L391

Signed-off-by: Marc Barussaud <marc.barussaud@orange.com>

Author: Marc Barussaud

utils/diff/diff.go

@@ -16,8 +16,8 @@ import (
 // string into the dst string.
 func Do(src, dst string) (diffs []diffmatchpatch.Diff) {
 	dmp := diffmatchpatch.New()
-	wSrc, wDst, warray := dmp.DiffLinesToChars(src, dst)
-	diffs = dmp.DiffMain(wSrc, wDst, false)
+	wSrc, wDst, warray := dmp.DiffLinesToRunes(src, dst)
+	diffs = dmp.DiffMainRunes(wSrc, wDst, false)
 	diffs = dmp.DiffCharsToLines(diffs, warray)
 	return diffs
 }