add gm repo bisect

[amtoine]

adds a new command to run a test: closure against a repo to bisect a new bug.
as can be seen in the documentation

• the test: closure should return non-zero exit codes for bad revisions
• the --good and --bad options should point to the first valid known good and bad revisions
• when the test: closure is expensive to run and --good and --bad are assument to be valid, their checks can be skipped with --no-check