Fix unified diff hunk header highlight to allow for single-number case

[sushicw]

Resolves #4227

Changes

Update the regex for matching a unified hunk header to allow for single numbers as well as comma-paired numbers.

From https://www.gnu.org/software/diffutils/manual/html_node/Detailed-Unified.html, emphasis mine:

If a hunk contains just one line, only its start line number appears. Otherwise its line numbers look like ‘start,count’. An empty hunk is considered to start at the line that follows the hunk.

If a hunk and its context contain two or more lines, its line numbers look like ‘start,count’. Otherwise only its end line number appears. An empty hunk is considered to end at the line that precedes the hunk.

Checklist

• Added markup tests, or they don't apply here because...
• Updated the changelog at CHANGES.md