fetch,pull: support literal path arguments - #6327

Summary

• allow git lfs fetch and git lfs pull to accept literal file or directory paths after --
• resolve relative and absolute paths against the repository while preserving filenames containing wildcard characters
• combine explicit paths with existing fetch include/exclude filters, and reject path filtering with fetch --all
• document the new syntax and add focused unit and integration coverage

Paths are matched literally rather than as gitignore patterns, so a filename such as literal[1].dat cannot accidentally select literal1.dat. Directory arguments include their descendants, and matching follows core.ignorecase where configured.

Tests

• make test
• make -C t test (105 test files, 835 tests)
• go vet ./commands ./filepathfilter

Closes #5946