Detect bisection to merge commits and to commits that don't affect
kernel binary (comments, other arches, whitespaces, etc).
Such bisections are not reported in emails (but shown on web).
Update #1271
* Modify Run() to return the commit object when a failure occurs on the
oldest tested release(for BisectCause) and latest tested
release(for BisectFix).
* Modify tests to check the returned commit object.
A bunch of improvements after more wide bisection testing.
Improve logging.
Support returning several commits for inconclusive bisection.
Return Report with the final crash.
Remove code that was moved to pkg/vcs.
Update #501
mgrconfig was used only by syz-manager initially,
but now it's used by a dozen of packages and it's
weird to import from under a binary dir.
pkg/ is much more reasonable dir for a widely used
helper package.
