add support for filtering results by passing `git diff` output

### Describe the desired feature

In CI, it'd be useful to only include the results of analyses introduced by changes in the PR
ref https://github.com/crytic/slither-action/issues/55

Start by tracing the analysis entry point and how CI consumes results, then determine where `git diff` output could be supplied. Done means CI can limit reported analyses to changes introduced by a pull request, with the expected behavior verified in the relevant tests.

Written by the indexing model from the issue text.