fix(git): refactor untracked file handling in getAllFileDiffs function

[FourWindff]

Description

Fixes untracked files not appearing in the diff viewer when multiple untracked files reside under a newly created
directory.

getAllFileDiffs previously used git status --porcelain to discover untracked files. When an entire directory
is untracked, Git collapses the output to a single directory entry (e.g. ?? src/components/) instead of listing
each individual file. The code treated this directory path as a file path, stat.isFile() returned false, and
every untracked file inside that directory was silently skipped — so they never appeared in the diff preview.

This change replaces git status --porcelain with git ls-files --others --exclude-standard, which always emits
one line per file regardless of directory structure. This aligns getAllFileDiffs with getChangedFiles, which
already uses the same ls-files command for untracked file detection.

Issues Resolved

• Untracked files in new directories (e.g. src/components/mdx/Blockquote.tsx) are now correctly included in the
  diff viewer output.
• Eliminates the discrepancy where the changed-files sidebar (powered by getChangedFiles) showed the files, but
  clicking them opened an empty diff because getAllFileDiffs had dropped them.

[johannesjo]

Thank you very much ! <3