Remove support for tilde fenced code blocks

CommonMark allows code fences using the tilde character in addition to
the backtick character. Commit 0a10f82 (Implement limited supported for
fenced code blocks, 2023-11-02) indicates that this is rarely used in
the wild, and in Linux 6.6 and Git 2.42.0 it is used almost exclusively
either to emphasize compilation error output or underline headers. The
variability in style does not serve commitmsgfmt, nor does the enormous
risk of false positives serve users, so remove tilde support entirely.

References: https://gitlab.com/mkjeldsen/commitmsgfmt/-/issues/7
References: https://spec.commonmark.org/0.30/#fenced-code-blocks

Author: commonquail

src/commitmsgfmt.rs

@@ -226,13 +226,6 @@ fenced
 code
 block
 ```
-
-~~~
-tilde
-fenced
-code
-block
-~~~
 ";
 
         let expected = "
@@ -244,13 +237,6 @@ fenced
 code
 block
 ```
-
-~~~
-tilde
-fenced
-code
-block
-~~~
 ";
 
         assert_eq!(filter(72, &input), expected);
@@ -266,12 +252,6 @@ a
 backtick
 ```
 b
-
-c
-~~~
-tilde
-~~~
-d
 ";
 
         let expected = "
@@ -282,12 +262,30 @@ a
 backtick
 ```
 b
+";
+
+        assert_eq!(filter(72, &input), expected);
+    }
+
+    #[test]
+    fn ignores_fenced_code_block_with_tilde() {
+        let input = "
+foo
 
-c
 ~~~
 tilde
+fenced
+code
+block
+not
+supported
 ~~~
-d
+";
+
+        let expected = "
+foo
+
+~~~ tilde fenced code block not supported ~~~
 ";
 
         assert_eq!(filter(72, &input), expected);