feat(setqflist): include untracked files with 'all' target

[nemanjamalesija]

Summary

• When setqflist('all') is called and attach_to_untracked is enabled, untracked files are now included in the quickfix list
• files_changed() accepts a new include_untracked parameter: uses ?? status from git status --porcelain (default base) or git ls-files --others --exclude-standard (custom base)
• Updated docs for the "all" target to document the new behavior

Closes #829

Test plan

• Set attach_to_untracked = true in config
• Create an untracked file in the repo
• Run :Gitsigns setqflist all and verify the untracked file appears in the quickfix list
• Run with attach_to_untracked = false and verify untracked files are excluded
• Test with a custom config.base to verify untracked files are included via ls-files

Assisted with claude code

[lewis6991]

Why did you remove the "Assisted with claude code" from the PR description?

[nemanjamalesija]

Why did you remove the "Assisted with claude code" from the PR description?

@lewis6991 I was not sure weather this would be something you would accept on the pull request

[lewis6991]

You should always include this disclosure.

[nemanjamalesija]

You should always include this disclosure.

Ok, I updated the comment