Show unchanged lines that fall inside an inline hunk

[ChrisJr404]

Closes #978.

Bug

When two novel lines in a hunk are close enough that the merge step in display::hunks::merge_adjacent combines them into one hunk, the unchanged lines between them are silently dropped from inline (--display=inline) output. Side-by-side and JSON output don't have this problem because both use matched_lines_indexes_for_hunk to walk the full hunk range, so they already include the in-between unchanged lines. Inline was iterating hunk.lines directly, and that field only contains positions with novel content.

In the report this surfaces as a missing section heading on a .gitconfig:

$ difft before.gitconfig after.gitconfig --display=inline

The line [url "ssh://git@github.com"] between two changed pushInsteadOf = lines does not appear.

Repro

before.gitconfig:

[uploadpack]
    allowFilter = true

[url "ssh://aur@aur.archlinux.org"]
    pushInsteadOf = "https://aur.archlinux.org"

[url "ssh://git@github.com"]
    pushInsteadOf = "https://github.com"

[versionsort]
    # prereleaseSuffix is deprecated in favour of suffix

after.gitconfig (same, but both pushInsteadOf lines are commented out):

[uploadpack]
    allowFilter = true

[url "ssh://aur@aur.archlinux.org"]
    ; pushInsteadOf = "https://aur.archlinux.org"

[url "ssh://git@github.com"]
    ; pushInsteadOf = "https://github.com"

[versionsort]
    # prereleaseSuffix is deprecated in favour of suffix

Before this PR

/tmp/after3.gitconfig --- Text
1    [uploadpack]
2        allowFilter = true
3    
4    [url "ssh://aur@aur.archlinux.org"]
5        pushInsteadOf = "https://aur.archlinux.org"
8        pushInsteadOf = "https://github.com"
   5     ; pushInsteadOf = "https://aur.archlinux.org"
   8     ; pushInsteadOf = "https://github.com"
   9 
   10 [versionsort]
   11     # prereleaseSuffix is deprecated in favour of suffix

Lines 6 (blank) and 7 ([url "ssh://git@github.com"]) are missing on both sides.

After this PR

/tmp/after3.gitconfig --- Text
1    [uploadpack]
2        allowFilter = true
3    
4    [url "ssh://aur@aur.archlinux.org"]
5        pushInsteadOf = "https://aur.archlinux.org"
6    
7    [url "ssh://git@github.com"]
8        pushInsteadOf = "https://github.com"
   5     ; pushInsteadOf = "https://aur.archlinux.org"
   6 
   7 [url "ssh://git@github.com"]
   8     ; pushInsteadOf = "https://github.com"
   9 
   10 [versionsort]
   11     # prereleaseSuffix is deprecated in favour of suffix

Fix

Add a small fill_in_hunk_gaps helper in src/display/inline.rs that walks hunk.lines and emits the intervening line numbers for each side, then update the LHS and RHS print loops to consult hunk.novel_lhs / hunk.novel_rhs so the new gap entries render with dimmed (context) line numbers instead of bold red / green ones.

I considered switching inline to use all_matched_lines_filled + matched_lines_indexes_for_hunk the same way side-by-side does, but that would also require restructuring how the before / after context loops are integrated. This narrower change keeps the existing structure and only fixes the dropped-context-inside-hunk case.

Testing

• Added four unit tests for fill_in_hunk_gaps covering the empty input, no-gap, LHS-only-changes-with-an-unchanged-tail, and unequal-side-gaps shapes.
• cargo test --release (148 tests) passes.
• Manually verified against the .gitconfig repro above and against syntactic (Rust) inputs to confirm the multi-hunk path still splits correctly when changes are far apart.