Bisect: implement "bisect skip" to mark untestable revisions.

Bisect: implement "bisect skip" to mark untestable revisions.

When there are some "skip"ped revisions, we add the '--bisect-all'
option to "git rev-list --bisect-vars". Then we filter out the
"skip"ped revisions from the result of the rev-list command, and we
modify the "bisect_rev" var accordingly.

We don't always use "--bisect-all" because it is slower
than "--bisect-vars" or "--bisect".

When we cannot find for sure the first bad commit because of
"skip"ped commits, we print the hash of each possible first bad
commit and then we exit with code 2.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>