fix(review): parse diff hunk headers that omit a side's line count

[kheiner]

Describe what this PR does / why we need it

Fixes a crash in generate_position2line_map when a diff hunk header does not include the line count for a single-line side (e.g. @@ -1 +1,2 @@). Without the fix you get crashes <C-j>/preview navigation in :Octo pr list.

Does this pull request fix one issue?

Fixes #1517

Describe how you did it

Widened the hunk-header regex to accept a missing ,<count> on either side.

Describe how to verify it

kheiner#1 reproduces the crash: a one-line addition to .gitignore (which already had exactly 1 line) produces the diff hunk @@ -1 +1,2 @@ and a review comment on it creates a thread with that diffHunk. Opening it with :Octo pr edit 1 kheiner/octo.nvim:

• Crashes without the fix
• Renders cleanly with it

Special notes for reviews

Checklist

• Passing tests and linting standards
• Documentation updates in README.md and doc/octo.txt (Not applicable)