RFC: Make git bisect submodule aware.

When git bisect discovers that the change set that creates the failure also contains a submodule change, that submodule should then be bisected starting with the good super modules code and working to a break in the sub module. If the change contains submodule change and super code changes than the bisection gets trickier, so we need some ideas on how to solve that search.

Example:

Super: A-B-C-D-E
Sub:   s-s-y-y-z

Where s is not required to be a parent of y, meaning that there might be 300 commits or just 1 between s and y in the submodule or they are disjoint then the bisecting should happen both routes into the submodule.

So the git bisect found that the offending commit is C

Now if the offending commit only changes 1 submodule than it is a normal bisect for the changed submodule.

If in commit C file foo.c changed and the submodule changed a bit more work is needed.

If foo.c is not dependent on a new feature from between s and y than we are good, otherwise I feel a bit of human intervention might be needed for marking a 'good' version in the submodule. Otherwise we have to search for a 'good' starting point, a binary bisection probably won't help much but probably would not hurt, since you need a 'good' commit to start the bisection from.

So for the not dependent on new feature/bugfix in the submodule.

B's foo.c and s -> good/bad? B's foo.c and y -> good/bad? C's foo.c and s -> good/bad? C's foo.c and y -> good/bad?

have fun bisecting.....

It might be good to interactively ask, for this bisection will file foo.c have an affect? If the file has an effect: Ask if C's foo.c will work with range s..y, if not what is the expected range for operation, we know that we need version u of submodule to compile C's foo.c. Or we have to search (linear from s) to where we can work. otherwise, just bisect the submodule.

Steve