fix: use second tag line for qformat alignment

qformat now trims the shared prefix against both tag lines instead of
 checking first_tags twice, which keeps the generated ? lines aligned when the
 second tag line has different leading spaces.
Add a regression test for the alignment case.
Rename the UTF-8 regression test to match the PR description.
This release stays on 0.5.1 because the UTF-8 intraline diff fix changes
 generated diff output for some inputs.
The public API is unchanged, but diff formatting behavior differs, so we
 treat this as a release-line bump.

07-002e4f71

Author: RandyMcMillan

README.md

@@ -8,7 +8,7 @@ Simply add difflib to your dependencies block in Cargo.toml
 
 ```rust
 [dependencies]
-difflib = "0.4.0"
+difflib = "0.5.1"
 ```
 
 ## Documentation