sagemathgh-36871: Fix a bug for .ci/create-changes-html.sh

    
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There is a bug in `.ci/create-changes-html.sh` which can result in
failure of doc-build workflow, as seen in sagemath#36861.

For instance, from a hunk in a diff
```diff
 <span class="go">(X0, X1)</span>
 </pre></div>
 </div>
@@ -428,8 +426,8 @@ Default to cyclotomic base ring.</p></li>
 <p>A tuple of integers.</p>
 <p>EXAMPLES:</p>
 <div class="highl
```
the script read `428` instead of reading `426`, which is the correct
line number of modified part of a file. This caused failures.

Tested in sagemath#36861.

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URL: sagemath#36871
Reported by: Kwankyu Lee
Reviewer(s):

.ci/create-changes-html.sh

@@ -69,12 +69,14 @@ for block in diff_blocks:
         count = 0
         for line in block.splitlines():
             if line.startswith('@@ -'):
-                line_number = int(re.search(r'@@ -(\d+)', line).group(1))
-                for i in range(line_number, -1, -1):
-                    if content[i].startswith('<'):
-                        count += 1
-                        content[i] = f'<span id="hunk{count}" style="visibility: hidden;"></span>' + content[i]
-                        break
+                search_result = re.search(r'@@ -(\d+),(\d+) \+(\d+),(\d+)', line)
+                if search_result:
+                    line_number = int(search_result.group(3))
+                    for i in range(line_number - 1, -1, -1):
+                        if content[i].startswith('<'):
+                            count += 1
+                            content[i] = f'<span id="hunk{count}" style="visibility: hidden;"></span>' + content[i]
+                            break
         with open(file_path, 'w') as file:
             file.writelines(content)
         hunks = '&nbsp;'.join(f'<a href="{path}#hunk{i+1}" class="hunk" target="_blank">#{i + 1}</a>' for i in range(count))