Scriptable mode for git bisect

Dear Git maintainers,

I have looked at the manpage of git bisect but have not found
what I need:
I would like git bisect not just to report the 'bad' revision
within a bunch of text but instead either stop at the first
bad revision (the last good will then be HEAD~1) or report
it in a scriptable way, i.e.

  BADHEAD=$(git bisect run --shut-up-and-report-the-bad)

Have I overlooked anything?

***

The pourpose is to keep the source trees of two different
projects that share a few files synchronous.
My good/bad-script is a script that checks whether these
files are similar.
I want git to stop at the first change in source tree A
so that I can update source tree B with the same commit
message, then proceed to the next change in A that
changes one of those files.

Regards,

Cluas