--diff-branch option to allow for scans of changed files only

[nickrayjones]

My git component has an extremely large code base and it takes hours to
scan all files in the repository. Our builds only perform incremental
changes, building only the changed source files. Because of this it is
only necessary to scan changed source files during the build process.

This enhancement saves hours of downtime on incremental builds only
changing a few source files out of thousands.

There is also a tweak to allow detect-secrets to run on z/OS by
acknowledging code 129 as a file not found code.

[nickrayjones]

@IBM/detect-secrets