fix(parser): exclude git diff metadata from neogit filename patterns

[barrettruth]

Problem

Git diff metadata lines like "new file mode 100644" and "deleted file
mode 100644" matched the neogit "new file" and "deleted" filename
patterns in the parser, corrupting the current filename and breaking
syntax highlighting for subsequent hunks.

Closes #120

Solution

Add negative guards so "new file mode" and "deleted file mode" lines
are skipped before the neogit filename capture runs. The guard must
evaluate before the capture due to Lua's and/or short-circuit semantics
— otherwise the and-operator returns true instead of the captured
string.

Added 16 parser tests covering all neogit filename patterns, all git
diff extended header lines that could collide, and integration scenarios
with mixed neogit status + diff metadata buffers.

[barrettruth]

@phanen here's the fix. the logic is low key a bit scuffed and may need to get organized in the future. but i tested it with your examples and it fixed it. let me know - i marked this pr as closing the issue.

I keep adding new tests but damn, you keep finding things :/ (well, it is good - just wish I thought of these things ahead of time).

Thanks again!

[barrettruth]