[271] Add flag to only scan files that exist on head

[AbsoLouie]

Related to Issue #271

[svishwanath-tw]

Great work on the PR. Almost completely self sufficient. (An update to the relevant section in the README would be highly appreciated).

This looks like a pre-cursor to scanning between 2 commits.
I'm wondering about the use case for scanning HEAD alone. Most modern CI's allow bunch of commits through in a single go unless it is explicitly set to run for every single commit.

[AbsoLouie]

I thought about that too, but I wasn't sure what the interface should actually look like for such a feature. git log has such a rich filtering mechanism and I was wondering if it would be better to just allow the user to enter the filter themselves instead of us creating it for them.

The other part is that my team currently uses feature branches so I was mostly concerned with setting up a stage gate on the PR itself. This assumes that if a secret is added and removed that the secret was properly rotated.

I think in both cases filtering would need to be combined with being able to ignore files. At the moment I can't create a green pipeline because of an issue that occurred at the start of the project and has since been resolved.