Search backend: fix multibyte line match conversion

Created by: camdencheek

This fixes a bug introduced by the multiline match code that causes the length for LineMatch to be incorrectly calculated as the number of bytes rather than the number of runes.

Before:

After:

The apostrophe in Don't is a multibyte character, which is what caused the breakage here.

Test plan

Added a test to check behavior for multibyte characters on internal lines.