src/scripts/config-diff.py: simplify sparse_branch_checkout_* functions and add files...

src/scripts/config-diff.py: simplify sparse_branch_checkout_* functions and add files names to POSIX diff

Refactored `sparse_branch_checkout_skip_clone` and
`sparse_branch_checkout_remote_repo_skip_clone` to accept and use
branch/tag names directly instead of constructing `ref_sha` strings
throughout the code.

Also include filenames from where the configuration values are coming
from in the POSIX diff. This helps identify the config options faster in
case of descrepancies.

Signed-off-by: Naveen Naidu <naveennaidu479@gmail.com>