src/script/config_diff.py: add support for `ref-commit-sha` and `cmp-commit-sha`...

src/script/config_diff.py: add support for `ref-commit-sha` and `cmp-commit-sha` arguments

Introduced `ref-commit-sha` and `cmp-commit-sha` arguments to the
`diff-branch-remote-repo` mode, enabling comparison of remote
branches against specific commits.

This enhancement is crucial for comparing configuration changes
between a pull request (PR) and the Ceph upstream main branch. It
allows for precise comparison by focusing on files changed in the
PR, rather than simply comparing the PR's head with its latest
commit.

The approach mirrors GitHub's three-dot diff [1], where the PR is
compared against the common ancestor of the Ceph upstream repository
, i.e., the point where the PR was forked.

[1]: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/about-comparing-branches-in-pull-requests

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