Merge branch 'cb/bisect-helper-parser-fix'

The code to parse "git bisect start" command line was lax in
validating the arguments.

* cb/bisect-helper-parser-fix:
  bisect--helper: avoid segfault with bad syntax in `start --term-*`

diff --cc builtin/bisect--helper.c
Simple merge

diff --cc t/t6030-bisect-porcelain.sh
Simple merge