Problem: Repeated default fields and field-by-field assertions obscure
the values that each functional test is meant to check.

Solution: Add eq_partial() to check selected record fields while
preserving list length and order. Use local helpers to fill fixed
expected defaults where tests need full equality. Keep explicit absence
checks and test-specific values at the call sites.

AI-assisted

Problem:
The magic globals `it`, `describe`, etc., are more trouble than they are
worth.

- Hooking into `after_each` requires `getfenv()` hacks.
- They confuse luals/emmylua, because the top-level `.luarc.json` isn't
  merged with `test/.luarc.json` (apparently a luals limitation?)
- They totally defeat discoverability because the user just has to
  "know" about the various magic symbols.

So they harm DX, which means they serve no purpose at all.

Solution:
- Expose the test API from `testutil`, so tests can call `t.it()`,
  `t.describe()`, etc., in the conventional way.
- Drop `getfenv()` hacks.
- Drop the `setfenv()` injection in `load_chunk`.
- Drop `test/_meta.lua`.

Problem:
Using the `DiffTool` plugin (e.g. through `nvim -d ...` or `:DiffTool
<file1> <file2>` fails if a space is in one of the paths. This occurs
because the `diff` wraps the paths with quotes (`'`) if space
characters are present, which the line diff regex fails to parse.

Solution:
Update regex to handle quoted paths by matching the string within the
quotes, if it exists.

Problem:
"nvim -d" doesn't leverage nvim.difftool.

Solution:
If nvim.difftool was enabled via :packadd, automatically
handle "nvim -d" on startup.

    nvim -c "packadd nvim.difftool" -d dir1/ dir2/

Closing the quickfix window previously triggered a WinClosed autocmd
that deleted all difftool autocmds and pushed an empty quickfix list,
making the difftool non-functional. Users who close the quickfix window
to gain screen real estate for viewing diffs had no way to continue
navigating entries.

Remove the qf_win tracking and its associated WinClosed autocmd so that
closing the quickfix window no longer tears down the difftool state.
Closing either diff window still performs full cleanup as before.

The BufWinEnter handler no longer passes with_qf to diff_files, so
navigating entries while the quickfix window is closed reuses the
existing diff layout without forcing a layout rebuild.

Fixes #37388

Fixes issue on mac where it was constantly reloading buffers as paths
were not being normalized and resolved correctly (in relation to buffer
name).

Quickfix entry:
/var/folders/pt/2s7dzyw12v36tsslrghfgpkr0000gn/T/git-difftool.m95lj8/right/app.vue

Buffer name:
/private/var/folders/pt/2s7dzyw12v36tsslrghfgpkr0000gn/T/git-difftool.m95lj8/right/app.vue

/var was synlinked to /private/var and this was not being properly
handled.

Also added lazy redraw to avoid too many redraws when this happens in
future and added test for symlink handling.

Signed-off-by: Tomas Slusny <slusnucky@gmail.com>

- Always open the right window to the right, regardless of 'splitright'
  setting, ensuring the left window is always leftmost.
- Use `vim.fn.systemlist` for diffr output to avoid manual splitting.
- Add a test to verify window layout consistency with 'splitright' and
  'nosplitright' options.
- Escape quotes in git difftool example properly.

Signed-off-by: Tomas Slusny <slusnucky@gmail.com>

Problem:
Built-in diff mode (nvim -d) does not support directory diffing
as required by git difftool -d. This makes it difficult to compare
entire directories, detect renames, and navigate changes efficiently.

Solution:
Add a DiffTool plugin and command that enables side-by-side diffing of
files and directories in Neovim. The plugin supports rename detection,
highlights changes in the quickfix list, and provides a user command for
easy invocation. This allows proper integration with git difftool -d for
directory comparison.

Example git config:

```ini
[diff]
    tool = nvim_difftool

[difftool "nvim_difftool"]
    cmd = nvim -c "packadd nvim.difftool" -c "DiffTool $LOCAL $REMOTE"
```

Signed-off-by: Tomas Slusny <slusnucky@gmail.com>
Co-authored-by: Phạm Bình An <111893501+brianhuster@users.noreply.github.com>
Co-authored-by: Justin M. Keyes <justinkz@gmail.com>