Use --exclude-from=.gitignore in place of --exclude-standard

[precondition]

Description

The lint check spews an error anytime it encounters a file that's listed in the output of git_get_ignored_files. However, the use of --exclude-standard in the git shell command ran by the aforementioned python function also ignores files in .git/info/exclude and the user's global .gitignore.

$ man git ls-files
[snip]
       -X <file>, --exclude-from=<file>
           Read exclude patterns from <file>; 1 per line.

       --exclude-per-directory=<file>
           Read additional exclude patterns that apply only to the directory and
           its subdirectories in <file>.

       --exclude-standard
           Add the standard Git exclusions: .git/info/exclude, .gitignore in
           each directory, and the user’s global exclusion file.
[snip]

This has the undesirable consequence of always failing the lint check if the user added some of his keyboard or keymap folders in a personal git ignore file like .git/info/exclude or his global .gitignore.

As explained in the docstring of the function, the intent of the function is only to "return a list of files that would be captured by the current .gitignore". To actually achieve that and avoid false negatives, using the option exclude-from=.gitignore is more appropriate.

Note: exclude-from=.gitignore works even when the check_dir is not the root directory

~/qmk_firmware on stricter_git_ignored_files *3 ⨤3                                
❯ cat .git/info/exclude
# git ls-files --others --exclude-from=.git/info/exclude
# Lines that start with '#' are comments.
# For a project mostly in C, the following would be a good set of
# exclude patterns (uncomment them if you want to use them):
# *.[oa]
# *~
keyboards/handwired/dactyl_manuform/5x6/keymaps/precondition
keyboards/polilla/rev1/keymaps/precondition
keyboards/splitography

~/qmk_firmware on stricter_git_ignored_files *3 ⨤3
❯ cd keyboards/handwired/dactyl_manuform/5x6

~/qmk_firmware/k/handw/dactyl_manuform/5x6 on stricter_git_ignored_files *3 ⨤3 
❯ touch via.json

~/qmk_firmware/k/handw/dactyl_manuform/5x6 on stricter_git_ignored_files *3 ⨤3    
❯ git ls-files -c -o -i --exclude-standard  
keymaps/precondition
via.json

~/qmk_firmware/k/handw/dactyl_manuform/5x6 on stricter_git_ignored_files *3 ⨤3    
❯ git ls-files -c -o -i --exclude-from=.gitignore
via.json

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).