pkg/bisect: support bisections on other trees

The current code only supports fix/cause bisections when the known bad
commit is reachable from Kernel.Repo/Kernel.Branch.

Add a CrossTree parameter to pkg/bisect. If it's set to true and we're
doing a fix bisection, the bisection algorithm first operates with the
original commit message (i.e. checks that it indeed crashes the kernel
and performs config minimization), but the actual bisection starts from
the merge base of Commit and Branch.

We could have calculated the merge base outside of pkg/bisect and just
started the algorithm from that merge base, but there's a problem:
there's no guarantee that the kernel will build/boot with a syzbot
config at the merge base. So we take the commit known to work well and
then assume that the bug is also present on the merge base commit. If it
were not present, we wouldn't have found a fix commit from Branch
anyway.

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